SC2620
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Dual Step-down Regulator with Programmable Frequency 1.4MHz
Wide Input Voltage Range 2.8V 1.4MHz/Channel Programmable Switching
Frequency Current-mode Control Phase Switching Reduces Ripple Cycle-by-cycle Current-limiting Independent Shutdown/soft-start Pins Independent Hiccup Overload Protection Channel Power Good Indicator 2.3A Integrated Switches Thermal Shutdown Thermally Enhanced SO-16 Lead Free Package Fully WEEE RoHS Compliant
SC2620
SC2620 constant frequency dual current-mode switching regulator with integrated 2.3A, switches. switching frequency programmed 1.4MHz channel. SC2620's high frequency operation, small inductors ceramic capacitors used, resulting very compact power supplies. channels SC2620 operate 180° phase reduced input voltage ripples.
Separate soft start/enable pins allow independent control each channel. Channel power good indicator used output start sequencing prevent latch-up. Current-mode control achieves fast transient
response with simple loop compensation. Cycle-by-cycle current limiting hiccup overload protection reduce power dissipation during overload.
Applications
XDSL Cable Modems Set-top Boxes Point Load Applications Equipment Power Supplies
Typical Application Circuit
BOOST1 1N4148 0.1µF ROSC 46.4k 9V-16V 10µF COMP2 33pF 10.5k 4.7nF BOOST2 1N4148 0.1µF 22µF 0.1µF UPS120 6.8µH OUT2 1.2V/2A 2.61k 10µH UPS120 22µF 30.1k
12.7k 1.5nF COMP1 47pF 22nF
OUT1 3.3V/2A
SC2620
PGOOD1 PVIN
22nF
4ms/div OUT1 Voltage, 2V/div OUT2 Voltage, 1V/div Voltage, 2V/div Figure 1(b). Start-up Transient (IOUT1= 1.5A, IOUT2= 0.8A). Channel start delayed until Channel reaches regulation.
Coiltr onics DR73
Murata GRM21BR60J 226M C15: Murata GRM32DR61E106K
Figure 1(a). 550kHz 9V-16V 3.3V 1.2V Stepdown Converter.
Revision: March 2009
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SC2620
POWER MANAGEMENT Absolute Maximum Ratings
Exceeding specifications below result permanent damage device, device malfunction. Operation outside parameters specified Electrical Characteristics section implied.
Parameter Input Voltage Boost Boost Above PGOOD1 Voltage Pins Pins Voltage Thermal Resistance Junction Ambient Thermal Resistance Junction Case Maximum Junction Temperature Storage Temperature Range Reflow Temperature Lead Temperature (Soldering)10 Rating (Human Body Model) (Note
Symbol VBST BST-V VPGOOD1 TSTG TLEAD
-0.3 -0.3 -0.6 +150
Units °C/W °C/W
Note This device sensitive. Standard handling precaution required.
Recommended Operating Conditions
Performance guaranteed exceeding specifications below
Parameter Input Voltage Range Ambient Temperature Range Junction Temperature
Symbol
Conditions
Units
Electrical Characteristics
Unless specified: -40°C 125°C, ROSC 12.1k, VBOOST
Parameter Start Voltage Start Hysteresis Quiescent Current Shutdown Current Feedback Voltage Feedback Voltage Regulation Input Bias Current
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Conditions
2.45
2.62
2.78
Units
switching, PGOOD1 Open SS1=V SS2=0, PGOOD1 Open 0.980 Vin=3V FB=1V, VCOMP=1.5V
1.000 0.005
1.020
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SC2620
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless specified: -40°C 125°C, ROSC 12.1k, VBOOST
Parameter Error Amplifier Transconductance Error Amplifier Open-Loop Gain COMP Source Current COMP Sink Current COMP Switch Current Gain COMP Switching Threshold COMP Maximum Voltage Channel Switching Frequency Maximum Duty Cycle Switch Current Limit Switch Saturation Voltage Switch Leakage Current Minimum Boost Voltage Boost Current Minimum Soft-Start Voltage Exit Shutdown Soft-start Charging Current Soft-start Discharging Current Minimum Soft-start Voltage Enable Overload Shutoff Overload Threshold Soft-start Voltage Restart Switching After Overload Shutoff Power Good Threshold Below Power Good Output Voltage Power Good Leakage Current Thermal Shutdown Temperature Thermal Shutdown Hysteresis
Conditions
Units
0.8V, VCOMP 1.5V 1.2V, VCOMP 1.5V
0.9V
(Note (Notes
(Note -0.5A Tied 1.5V 1.5V Rising 2.3V, Falling Falling VFB1 Rising VFB1 0.8V, IPGOOD1 250µA VPGOOD1
Note Guaranteed design, 100% tested production. Note maximum duty cycle specified corresponds 1.4MHz switching frequency. Duty cycles higher than those specified achieved lowering operating frequency. Note Switch current limit does vary with duty cycle.
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SC2620
POWER MANAGEMENT Configuration
VIEW
BOOST1 PVIN1 PVIN2 BOOST2
Ordering Information
Part Number
COMP1 PGOOD1 ROSC COMP2
Package SOIC-16 Evaluation Board
SC2620SETRT(1)(2) SC2620EVB
Notes: Only available tape reel packaging. reel contains 2500 devices. Lead free product. This product fully WEEE RoHS compliant.
SOIC-EDP) Underside metal must soldered ground.
Descriptions
Name FB1, BOOST1, BOOST2 SW1, Function inverting inputs error amplifiers. Each tied resistive divider between output ground channel output voltage. Supply pins power transistor drivers. external diode-capacitor charge pumps generate drive voltages higher than order fully enhance internal power switches. Emitters internal power transistors. Each connected corresponding inductor, freewheeling diode bootstrap capacitor.
Collectors internal power transistors power supplies corresponding current PVIN1, PVIN2 sensing circuits. Pins internally connected. They must joined closely bypassed power ground plane. COMP1, COMP2 Outputs internal error amplifiers. voltages these pins control peak switch currents. networks these pins stabilize control loops. Pulling either below 0.7V stops corresponding switching regulator. capacitor from either ground provides soft-start overload hiccup functions that channel. Pulling either below 0.8V with open drain collector transistor shuts corresponding regulator. completely shut SC2620 low-current state, pull both pins ground. Soft-start recommended applications. Analog ground. Connect power ground plane single point. Power supply analog control section SC2620. Connect PVIN pins through optional filter. external resistor between this analog ground sets channel switching frequency. Open collector output Channel power good comparator. external pull-up resistor from input output converter. PGOOD1 output becomes valid soon rises above during power-up. PGOOD1 actively pulled until voltage rises within final regulation voltage. exposed bottom package electrically connected ground SC2620. also serves thermal contact circuit board. soldered analog ground plane board.
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SS1,
ROSC
PGOOD1
Underside Metal
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SC2620
POWER MANAGEMENT Block Diagrams
PVIN1
PGOOD1
CHANNEL ONLY POWER GOOD 100mV
ISEN 6.3m ILIM 20mV
SLOPE COMP
COMP1
BOOST1
POWER TRANSISTOR
0.7V FAULT
REFERENCE THERMAL SHUTDOWN SLOPE COMP
Soft-Start Overload Hiccup Control
OVLD
SLOPE COMP
ROSC
SLOPE COMP OSCILLATOR CLK1 FREQUENCY CLK2 DIVIDER
Figure SC2620 Block Diagram (Channel
0.7V
1.8µA
1V/2V OVLD
FAULT 2.6µA
Figure Details Soft-Start Overload Hiccup Control Circuit
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SC2620
POWER MANAGEMENT Typical Characteristics
Feedback Voltage Temperature
1.02 1.01 1.00 0.99 0.98 0.97 Temperature (°C)
Frequency Setting Resistor Channel Frequency
1000
Normalized Frequency 1.05 1.10
Normalized Channel Frequency Temperature
600kHz
ROSC
1.00 1.4MHz 0.95
0.90
Frequency
Temperature (°C)
Switch Saturation Voltage Switch Current
125°C
Switch Current Limit Temperature
Current Limit
Boost Current Switch Current
Boost Current (mA)
VCESAT (mV)
-40°C
-40°C
125°C
25°C Switch Current Temperature (°C)
Switch Current
Shutdown Threshold Temperature
0.40
VSS1 VSS2
Shutdown Current
25°C
Current
Current (mA)
Quiescent Current
Threshold
0.35
0.30
0.25
VSS1 VSS2
25°C
0.20 Tempe rature (°C)
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SC2620
POWER MANAGEMENT Typical Characteristics
Soft-Start Current Soft-Start Voltage
25°C -100 -120 Other Channel (mA) Swept Channel 25°C
Threshold
Supply Current Soft-Start Voltage
Overload Threshold Temperature
VSS1 COMP1 COMP2
Temperature (°C)
PGOOD1 Threshold Difference Voltage Temperature
Efficiency Load Current
Voltage
Efficiency
VOUT1 3.3V
VOUT2 1.2V
Figure 1(a), VIN=12V
-100 Temperature (°C)
Load Current
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SC2620
POWER MANAGEMENT Operation
SC2620 2-channel constant-frequency peak current-mode step-down switching regulator with integrated 2.3A power transistors. Both regulators SC2620 operate from common input power supply share same voltage reference master oscillator. Turn-on power transistors phase-shifted 180°. regulator cores otherwise completely identical, independent capable producing separate outputs from same input. channel frequency programmed with external resistor from ROSC ground. This allows designer switching frequency according input output voltage conversion ratio. Peak current-mode control utilized SC2620. double reactive poles output filter reduced single real pole inner current loop, easing loop compensation. Fast transient response achieved with simple Type-2 compensation network. Switch collector current sensed with integrated 6.3m sense resistor. sensed current summed with slopecompensating ramp before compared with transconductance error amplifier output. comparator tripping point determines switch turn-on pulse width (Figure current-limit comparator ILIM turns power switch when sensed-signal exceeds 20mV current-limit threshold. ILIM therefore provides cycle-by-cycle limit. Current-limit does vary with dutycycle. external charge pump (formed capacitor diode Figure 1(a)) generates voltage higher than input rail BOOST pin. bootstrapped voltage generated becomes supply voltage power transistor driver. Driving base power transistor above input power supply rail minimizes power transistor turn-on voltage maximizes efficiency. multiple-function pin. external capacitor connected from ground together with internal 1.8µA 2.6µA current sources softstart overload shutoff times regulator (Figure also used shut corresponding regulator. When either pulled below 0.8V, that regulator turned off. both pins pulled below 0.2V, then SC2620 undergoes overall shutdown. current drawn from input power supply reduces 40µA. When either released, corresponding soft-start capacitor charged with current source (not shown Figure either voltage exceeds 0.3V, internal bias circuit SC2620 enabled. SC2620 draws 3.5mA from VIN. internal fast charge circuit quickly charges soft-start capacitor this juncture, fast charge circuit turns 1.8µA current source slowly charges soft-start capacitor. output error amplifier forced track slow soft-start ramp pin. When COMP voltage exceeds 1.1V, switching regulator starts switch. During soft-start, current limit converter gradually increased until converter output comes into regulation. Hiccup overload protection utilized SC2620. Overload shutdown disabled during soft-start (VSS 2V). Figure reset input overload latch will remain high voltage below Once soft-start capacitor charged above overload shutdown latch enabled. load draws more current from regulator, current-limit comparator will limit peak inductor current. This cycle-by-cycle current limiting. Further increase load current will cause output voltage decrease. output voltage falls below point, then overload latch will soft-start capacitor will discharged with current 0.8µA. switching regulator shut until softstart capacitor discharged below this moment, overload latch reset. soft-start capacitor recharged converter again undergoes soft-start. regulator will through soft-start, overload shutdown restart until longer overloaded. power good comparator indicates that channel regulator output risen within value. open collector output power good comparator will actively pulled feedback voltage below 0.9V.
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SC2620
POWER MANAGEMENT Applications Information
Setting Output Voltage regulator output voltage with external resistive divider (Figure with center tied pin.
VOUT
Channel switching frequency limited minimum controllable time duty cycles. 20V, setting switching frequency below 500kHz makes converter output short circuit operation more robust. These will described more details later. Minimum Time Consideration
15nA
SC2620
operating duty cycle non-synchronous step-down switching regulator continuous-conduction mode (CCM) given
Figure VOUT with Resistive Divider (VOUT
VOUT VCESAT
where VCESAT switch saturation voltage voltage drop across rectifying diode. Duty cycle decreases with increasing
percentage error input bias current error amplifier VOUT 15nA VOUT Example: Determine output voltage error VOUT converter with 51.1k From (1), 51.1k 205k VOUT 15nA (51.1k205k) -0.061% VOUT
ratio. peak VOUT
current-mode control, modulating ramp sensed current ramp power switch. This current ramp absent unless switch turned intersection this ramp with output voltage feedback error amplifier determines switch pulse width. propagation delay time required immediately turn switch after turned minimum controllable switch time (MIN) Closed-loop measurement SC2620 with
VOUT ratios shows
Minimum Time Ambient Temperature
TON(MIN) (ns)
This error least order magnitude lower than ratio tolerance resulting from resistors divider string. Setting Channel Frequency switching frequency master oscillator with external resistor from ROSC ground. Channel frequency one-half that master oscillator. graph channel frequency against ROSC shown "Typical Performance Characteristics". Channel frequency programmable 1.4MHz.
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Temperature (°C)
Figure Variation Minimum Time with Ambient Temperature.
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SC2620
POWER MANAGEMENT Applications Information
that minimum time about 105ns room temperature (Figure power switch SC2620 either turned least TON(MIN). Example: Determine maximum operating frequency dual 1.5V switching regulator using SC2620. Assuming that 0.45V, VCESAT 0.25V 4.5V (10% line), duty ratios 1.5V converters calculated using (2).
shorter than minimum time, regulator will either skip cycles will jitter.
required switch time Example: Determine maximum operating frequency dual 1.2V 3.3V switching regulator using SC2620. Assuming that 0.45V, VCESAT 0.25V 26.4V (10% high line), corresponding duty ratios, 1.2V 3.3V converters calculated using (2).
0.45 0.42 0.45 0.25 0.45 0.95 0.45 0.25
maximum operating channel frequency dual
0.45 0.062 26.4 0.45 0.25 0.45 0.14 26.4 0.45 0.25
1.5V converter therefore
410kHz 120ns
Transient headroom requires that channel frequency lower than 410kHz. Inductor Selection inductor ripple current non-synchronous stepdown converter continuous-conduction mode
VOUT VOUT VOUT VCESAT VCESAT
allow transient headroom, minimum operating switch time should least higher than worst-case minimum time exhibited Figure Designing switch time 150ns 26.4 maximum operating frequency 1.2V 3.3V converter
410kHz 150ns
Minimum Time Limitation where switching frequency inductance. latch Figure reset every period clock. clock also turns power transistor refresh bootstrap capacitor. This minimum time limits attainable duty cycle regulator given switching frequency. measured minimum time 120ns. step-down converter, increases with increasing current-mode control, slope modulating (sensed switch current) ramp should steep enough lessen jittery tendency steep that large flux swing decreases efficiency. Inductor ripple current between 25-40% peak inductor current limit good compromise. Inductors chosen optimized size DCR. Setting 0.3(2.3) 0.69
VOUT
ratio. required duty cycle higher than attainable maximum, then output voltage will able reach value continuous-conduction mode.
0.45 VCESAT 0.25 (3),
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SC2620
POWER MANAGEMENT Applications Information
0.45)( VOUT 0.25) 0.2)(0.69) where MHz. Equation shows that given VOUT increases decreases. varies over wide range, then choose based nominal input voltage. Always verify converter operation input voltage extremes. peak current limits both SC2620 power transistors internally 3.2A. peak current limits dutycycle invariant guaranteed higher than 2.3A. maximum load current therefore conservatively:
IOUT (MAX
Power dissipated input capacitor IRMS( CIN) (ESR)
Equation maximum value CIN.
IOUT corresponding worst-case power dissipation
dual-channel step-down converter with interleaved switching reduces ripple current input capacitor fraction that single-phase buck converter. both power transistors SC2620 were switch phase, current drawn SC2620 would consist current pulses with amplitude equal channel output currents. each channel were delivering IOUT operating duty cycle, then input current would switch from zero 2IOUT. ripple current input capacitor would then IOUT. Power dissipated would times that single-channel converter. SC2620 produces highest ripple current when only channel running delivering maximum output current (2A). input capacitor therefore should have ripple current rating least Multi-layer ceramic capacitors, which have very easily handle high ripple current, ideal choice input filtering. single 4.7µF 10µF ceramic capacitor adequate. high voltage applications, small ceramic (1µF 2.2µF) placed parallel with electrolytic capacitor satisfy both bulk capacitance requirements. Output Capacitor output ripple voltage VOUT buck converter expressed VOUT fCOUT where COUT output capacitance. Inductor ripple current increases decreases (Equation (3)). output ripple voltage therefore highest when maximum. first term results from output capacitor while
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then IOUT(MAX 0.3ILM 0.85
saturation current inductor should 20-30% higher than peak current limit (2.3A). Low-cost powder iron cores suitable high-frequency switching power supplies their high core losses. Inductors with ferrite cores should used. Power Line Input Capacitor buck converter draws pulse current with peak-to-peak amplitude equal output current IOUT from input supply. input capacitor placed between supply buck converter filters current keeps current drawn from supply constant. input capacitance should high enough filter pulse input current. equivalent series resistance (ESR) should that power dissipated capacitor does result significant temperature rise degrade reliability. single channel buck converter, ripple current input capacitor IRMS( CIN) IOUT
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SC2620
POWER MANAGEMENT Applications Information
second term charging discharging COUT inductor ripple current. Substituting 0.69A, 500kHz COUT 22µF ceramic with (7), VOUT 0.69 11.4m) 1.4mV 7.8mV 9.2mV Depending operating frequency type capacitor, ripple voltage resulting from charging discharging COUT higer than that ESR. 10µF 47µF ceramic capacitor found adequate output filtering most applications. Ripple current output capacitor concern because inductor current buck converter directly feeds COUT, resulting very ripple current. Avoid using ceramic capacitors output filtering because these types capacitors have high temperature high voltage coefficients. Freewheeling Diode Schottky barrier diodes freewheeling rectifiers reduces diode reverse recovery input current spikes, easing high-side current sensing SC2620. These diodes should have average forward current rating between reverse blocking voltage least volts higher than input voltage. switching regulators operating duty cycles (i.e. output voltage input voltage conversion ratios), beneficial freewheeling diodes with somewhat higher average current ratings (thus lower forward voltages). This because diode conduction interval much longer than that transistor. Converter efficiency will improved voltage drop across diode lower. freewheeling diodes should placed close pins SC2620 minimize ringing trace inductance. 10BQ015, 20BQ030 (International Rectifier), MBRM120LT3 Semi), UPS120 UPS140 (MicroSemi) suitable. Bootstrapping Power Transistors maximize efficiency, turn-on voltage across internal power transistors should minimized. these transistors driven into saturation, then
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their bases will have driven from power supply higher voltage than VIN. required driver supply voltage least 2.5V higher than voltage over industrial temperature range) generated with bootstrap circuit (the diode DBST capacitor CBST Figure bootstrapped output (the common node between DBST CBST) connected BOOST SC2620. power transistor SC2620 first switched build current inductor. When transistor switched off, inductor current pulls node low, allowing CBST charged through DBST. When power switch again turned voltage goes high. This brings BOOST voltage thus back-biasing DBST. CBST voltage increases with each subsequent switching cycle, does bootstrapped voltage BOOST pin. After number switching cycles, CBST will fully charged voltage approximately equal that applied anode BST. Figure shows typical minimum BOOST voltage required fully saturate power transistor. This differential voltage must least 1.8V room temperature. This also specified "Electrical Characteristics" "Minimum Bootstrap Voltage". minimum required increases temperature decreases. bootstrap circuit reaches equilibrium when base charge drawn from during transistor time equal charge replenished during interval.
Minimum Bootstrap Voltage Temperature
Voltage
Temperature (°C)
Figure Typical Minimum Bootstrap Voltage Required Maintain Saturation
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SC2620
POWER MANAGEMENT Applications Information
switch base current where switch emitter current current gain respectively, drawn from bootstrap capacitor CBST. Charge drawn from CBST during switch time, resulting voltage droop 1µs, refreshed VDBST VDRECT every cycle, where applied DBST anode voltage. Switch base current discharges bootstrap capacitor VDBST VDRECT CBST
conduction. This voltage must higher than minimum shown Figure ensure full switch enhancement. DBST tied either input output DC/DC converter. DBST tied input, then charge drawn from
VBST 2VIN
CBST 0.1µF, then VCBST droop will 0.57V. CBST
VBST VOUT
DBST
DBST
BOOST
CBST VOUT
BOOST
CBST VOUT
SC2620
RECT
SC2620
DRECT
DBST
VBST 2VIN
2.5V
DBST
VBST
BOOST CBST VOUT RECT BOOST CBST VOUT
SC2620
SC2620
DRECT
2.5V DBST
BOOST VOUT
SC2620
RECT
Figure Methods Bootstrapping SC2620.
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SC2620
POWER MANAGEMENT Applications Information
(the base charge switch). energy loss base charge cycle input power supply will DISW VOUT power loss DBST tied output, then charge drawn from output capacitor will still base charge cycle loss DISW VOUT energy loss VOUT power BOOST pin. maximum BOOST voltage about VOUT output below 2.8V, then DBST will preferably small Schottky diode (such BAT-54) maximize bootstrap voltage. 0.33-0.47µF bootstrap capacitor needed reduce droop. Bench measurement shows that using Schottky bootstrapping diode noticeable efficiency benefit. SC2620 also bootstrapped from input (Figure 7(b)). This configuration efficient Figure 7(a). However this only option output voltage less than 2.5V there other supply with voltage higher than 2.5V. Voltage stress BOOST somewhat higher than 2VIN. Zener diode Figure 7(c) reduces maximum BOOST voltage. BOOST voltage should exceed absolute maximum rating 42V. Figures 7(d) show bootstrap SC2620 from second power supply with voltage 2.5V. Figure 7(d) output other channel. Figures 1(a), 17(a) 18(a) show this bootstrapping method. Channel fails these converters, Channel will shut (See Sequencing Outputs). Proper bootstrapping Channel therefore depends readiness VOUT1. This drawback some applications. DBST Figure 7(e) prevents start difficulty comes before
Since VOUT DBST should always tied VOUT >2.5V) maximize efficiency. general efficiency penalty increases decreases. Figure summarizes various ways bootstrapping SC2620. fast switching diode (such 1N4148 1N914) small (0.1µF 0.47µF) ceramic capacitor used. Figure 7(a) power switch bootstrapped from output. This most efficient configuration also results least voltage stress
Minimum Starting Sustaining Load Current
Minimum Input Voltage Minimum Input Voltage
STARTING DBST TIED OUTPUT MA729
Minimum Starting Sustaining Load Current
DBST TIED OUTPUT 3.3V MA729
STARTING
1000 Load Current (mA)
DBST TIED INPUT SUSTAINING
DBST TIED INPUT
SUSTAINING
10.0 100.0 1000.0 Load Current (mA)
Figure Minimum Input Voltage Required Start Maintain Bootstrap.(TA 25°C).
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SC2620
POWER MANAGEMENT Applications Information
Since inductor current charges CBST, bootstrap circuit requires some minimum load current going. Figures 8(a) 8(b) show dependence minimum input voltage required properly bootstrap 3.3V converters load current. Once started bootstrap circuit able sustain itself down zero load. Shutdown Soft-Start Each regulating channel SC2620 softstart circuit. Pulling soft-start below 0.8V with open-collector open-drain NMOS transistor turns corresponding regulator. other regulator continues operate. With channel turned off, internal bias circuit kept alive. "Typical Characteristics", soft-start current plotted against soft-start voltage with When softstart pins pulled low, 105µA flows that pin. Pulling both soft-start pins below 0.2V shuts internal bias circuit SC2620. total current decreases 40µA. shutdown either sources only 2µA. fast charging circuit (enabled internal bias circuit), which charges soft-start capacitor below causes difference soft-start currents. either released shutdown, internal current source pulls pin. When this voltage reaches 0.3V, SC2620 turns quiescent current
2.4V 0.3V 0.7V Switching Starts
Hiccup Enabled
Fast Charge
Output must least voltage this interval regulator will undergo shutdown restart (hiccup).
Figure 9(a). Normal Soft-start.
VCOMP 0.3V Switching Switching Switching Switching
0.7V
Figure 9(b).
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Start-up Fails Short Soft-start Duration (ii) Output Overload (iii) Output Short-circuited.
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SC2620
POWER MANAGEMENT Applications Information
increases 3.3mA. current flowing other (which still pulled low) increases 105µA. fast charging circuit quickly pulls released soft-start capacitor (slightly below switching threshold). fast charging circuit then disabled. 1.8µA current source continues charge soft-start capacitor (Figure soft-start voltage ramp clamps error amplifier output (Figure During regulator startup, COMP voltage follows voltage. converter starts switch when COMP voltage exceeds 1.1V. peak inductor current gradually increases until converter output comes into regulation. Proper soft-start prevents output overshoot during start-up. Current drawn from input supply also well controlled. Notice that inductor current, converter output voltage, ramped during soft-start. Both soft-start capacitors charged final voltage about 2.4V. Overload Short-Circuit Protection Each current limit comparator SC2620 limits peak inductor current 3.2A (typical). regulator output voltage will fall load increased above current limit. overload detected (the output voltage falls below voltage), then regulator will shut off. internal 0.8µA current sink starts discharge soft-start capacitor. soft-start capacitor discharged below discharge current source turns soft-start capacitor recharged with 1.8µA current source. regulator undergoes softstart. During soft-start 2V), overload shutdown latch Figure cannot set. When exceeds input overload latch longer blanked. still below 0.7V, then regulator will undergo shutdown restart. soft-start process should allow output voltage reach final value before charged above Figures 9(a) 9(b) show timing diagrams successful failed start-up waveforms respectively. soft-start interval should also made sufficiently long that output voltage rises monotonically does overshoot final voltage more than During normal soft-start, both COMP voltage switch current limit gradually increase until converter becomes regulated. regulator output shorted
PGOOD1
CONTROL1
CSS1
SC2620
CSS1
SC2620
PGOOD1
CONTROL2
CSS2
CSS2
CONTROL1 CONTROL2
Figure Sequencing Outputs Delaying Release Channel Relative Other Using PGOOD1 Control Channel
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SC2620
POWER MANAGEMENT Applications Information
ground, then COMP voltage will continue rise 2.4V upper limit. SC2620 will reach cycle-by-cycle current limit sometime during soft-start charging phase (see Figure 17(c)). described previously, switches SC2620 either turn least 105ns. With output shorted, error amplifier will command regulator operate full duty cycle. current limit comparator will turn switch switch current exceeds 3.2A. However, this happens only after switch turned 105ns. During switch time, inductor current ramps down slow rate determined forward voltage freewheeling diode resistance short. resulting reverse volt-second insufficient reset inductor before start next cycle, then inductor current will keep increasing until diode forward voltage becomes high enough achieve volt-second balance. This makes current limit comparator ineffective. Short circuit robustness will enhanced switching frequency below 500kHz high 20V). This increases time keeps inductor current within bounds. regulator checked under realistic short circuit condition residual resistance short significantly influence circuit behavior. Shortening soft-start interval from onset switching hiccup enable also makes short circuit operation more robust. 22-47nF soft-start capacitor found adequate most applications. Figure 17(c), Channel undergoes repeated shutdown restart ("hiccup") with output shorted. appears asymmetrical triangular wave. resistance short appears 17m. Power Good Indicator PGOOD1 (Pin open-collector output Channel power good comparator. This slow comparator incorporated with small amount hysteresis. low-to-high trip voltage power good comparator final regulation voltage. pull-up resistor from PGOOD1 input supply regulator output sets logic high level comparator. power good comparator output becomes valid provided that above 0.9V. shutdown power good output actively pulled low. power good pull-up resistor tied input will therefore increase current drain during shutdown. Tying power good pull-up resistor regulator output preferred, this will minimize
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shutdown supply current. shutdown there voltage switching regulator output current PGOOD1 pull-up resistor. PGOOD1 output high level VOUT) unacceptably low, then power good pull-up from input separate power supply will only choice. Sequencing Outputs mentioned above, pulling either soft-start with external transistor shuts corresponding regulator (Figure 10). Releasing soft-start enables that channel allows start. Delaying release soft-start channel with respect other straightforward sequencing outputs. Figure 10(a) shows this method using external transistors turned first, allowing channel start. Channel then enabled after time PGOOD1 also used conjunction with Channel soft-start delay start that regulator. This method depicted Figure 10(b). pulled channel kept until channel output rises voltage. Loop Compensation Figure shows simplified equivalent circuit stepdown converter. power stage, which consists current-mode comparator, power switch, freewheeling diode inductor, feeds output network. power stage modeled voltagecontrolled current source, producing output current proportional controlling input COMP transconductance 8-1. With current loop closed, control-to-output transfer function COMP
dominant-pole located frequency slightly higher than that output filter pole. nIOUT VOUT ROUT
where output capacitor, ROUT equivalent load resistance (depending duty ratio, slope compensation, frequency passive components) usually between
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SC2620
POWER MANAGEMENT Applications Information
POWER STAGE
VOUT ROUT
COMP
280µ 1.6M
VOLTAGE REFERENCE
Figure Simplified Control Loop Equivalent Circuit
ceramic, then zero neglected situates well beyond half switching frequency. frequency gain control-to-output transfer function simply product power stage transconductance equivalent load resistance (Figure 12). transfer functions feedback network error amplifier are: sC11R1 R1R2
addition form zero with angular frequency:
output-to-control
transfer
function
COMP COMP also shown Figure midv band gain (between GMAR5 overall loop gain T(s) product control-to-output output-to-control transfer functions. simplify
Bode plot, feedback network assumed
COMP GMARO sC6R5
(10)
provided that Equation (10), forms frequency pole with output resistance error amplifier forms high frequency pole with Amplifier Open Loop Gain 53dB 1.6M Transconduc 280µ
resistive. overall loop gain cross -20dB/ decade, then mid-band gain (between will tenth switching frequency
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SC2620
POWER MANAGEMENT Applications Information
This also equal GMPROUT GMAR5 Therefore
shown less than Figure Making
Re-arranging,
gives first-order estimate
(12)
10nGMP
(11)
Notice that determines mid-band loop gain converter. Increasing increases mid-band gain crossover frequency. However reduces phase margin. small ceramic capacitor roll loop
Gain
COMP
Control-to-Output Transfer Function
Figure Bode Plots Control-to-Ouput, Output-to-Control Overall Loop Gain. Control-to-output transfer function shown with poles near half switching frequency
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SC2620
POWER MANAGEMENT Applications Information
gain high frequency. Placing about
gives: (13)
Example: Determine compensation components 550kHz 9V-16V 3.3V 1.2V converter Figure 1(a). both channels, Mrads IOUT(MAX 22µF assumed (11) (12). 3.3V output: 30.1 (2.8 11.3 11.3 47pF (550 (11.3
Computed indeed result near optimal load transient responses over half applications. However other cases empirically determined compensation networks based optimized load transient responses differ from those calculated factor Therefore checking transient response converter imperative. Starting with calculated (using Equations (11)-(13)), apply largest expected load step converter maximum operating VIN. Observe load transient response converter while adjusting Choose largest smallest that inductor current waveform does show excessive ringing overshoot (see Figures 13(a), 13(b), 16(b) 16(c)). Feedforward capacitor boosts phase margin over limited frequency range sometimes used improve loop response. will more effective R1R2
VIN=16V VOUT=3.3V
VIN=16V VOUT=1.2V
40µs/div Upper Trace OUT1 Voltage, Coupled, 0.5V/div Lower Trace Inductor Current, 0.5A/div
40µs/div Upper Trace OUT2 Voltage, Coupled, 0.5V/div Lower Trace Inductor Current, 0.5A/div
Figure Load Transient Response Dual DC-DC Converter Figure 1(a). IOUT1 IOUT2 switched between 0.3A
2008 Semtech Corp.
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SC2620
POWER MANAGEMENT Applications Information
1.2V channel: 2.61 (2.8 4.12 4.12 150pF (550 (4.12
Board Layout Considerations step-down switching regulator, input bypass capacitor, main power switch freewheeling (Figure 14). jitter-free operation, size loop formed these components should minimized. Since power switches already integrated within SC2620, connecting anodes both freewheeling diodes close negative terminal input bypass capacitor minimizes size switched current loop. input bypass capacitors should placed close PVIN pins. Shortening traces BOOST nodes reduces parasitic trace inductance these nodes. This only reduces also decreases switching voltage spikes these nodes. diode carry discontinuous currents with high PVIN bypass capacitor output filtering capacitors freewheeling diodes grounded power ground plane (Figure 15). feedback resistive dividers, compensation networks, soft-
Bench measurement shows that compensation components computed from simplified linear model give very good load transient response Channel (Figure 13(a)). However, optimizing load transient Channel will require compensation component values different from those calculated above. Loop compensation networks shown Figure 1(a) empirically optimized load transients. Figures 13(a) 13(b) show corresponding load transient responses.
VOUT
Figure Fast Switching Current Paths Buck Regulator. Minimize size this loop reduce parasitic trace inductance.
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SC2620
POWER MANAGEMENT Applications Information
start capacitors filtering capacitor tied analog ground. frequency-setting resistor placed next ROSC also connected analog ground. resistor that connects analog ground power ground single point. exposed should soldered large analog ground plane analog ground copper acts heat sink device. ensure proper adhesion ground plane, avoid using vias directly under device. figure 12mil vias placed edge underside pad.
OUT1
OUT1
AGND
OUT2
Figure Suggested Layout SC2620.
2008 Semtech Corp.
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SC2620
POWER MANAGEMENT Typical Application Circuits
15.8k 22nF ROSC 15.0k PGOOD1 22nF 9.76k COMP2 390pF BOOST2 1N4148 0.1µF 22µF 10.7k 0.1µF 10BQ 1.8µH 2.15k OUT2 1.2V/2A PVIN 10µF COMP1 390pF BOOST1 0.1µF 1N4148 1.8µH 10BQ 10µF 24.9k 10.7k OUT1 3.3V/2A
Efficiency
Efficiency VOUT2 1.2V VOUT1 3.3V
SC2620
10pF
Wurth 0018 C15: Murata GRM21BR60J106K Murat GRM21BR60J 226M
Load Current
Figure 16(a). 1.2MHz 3.3V 1.2V xDSL Power Supply. Channel does start until Channel output voltage becomes regulated.
OUT1
OUT2
40µs/div Upper Trace OUT1 Voltage, Coupled, 0.5V/div Lower Trace Inductor Current, 0.5A/div
40µs/div Upper Trace OUT2 Voltage, Coupled, 0.2V/div Lower Trace Inductor Current, 0.5A/div
Figures 16(b) 16(c). Load Transient Response. IOUT switched between 0.3A
2008 Semtech Corp.
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SC2620
POWER MANAGEMENT Typical Application Circuits
0.1µF 47nF ROSC 51.1k 22nF COMP2 8.25k 22pF BOOST2 1N4148 0.1µF PGOOD1 0.1µF UPS120 OUT2 4.7µH 47µF 8.06k 0.8V/2A 4.02k PVIN 10µF 1N4148 UPS120 15µH 22µF 60.4k 15.0k 47pF OUT1 5V/2A 80.6k
15.4k
COMP1 BOOST1
SC2620
47pF
47pF
Coiltronics DR74 Coiltronics DR73
Murata GRM21BR60J 226M Murata GRM31CR60J476M C15: Murata GRM32DR61E106K
Figure 17(a). 500kHz 0.8V step-down converter. Notice that VOUT2 lower than nominal voltage. constitute feedback voltage divider Channel
4ms/div VIN, 5V/div OUT1 Voltage, 2V/div OUT2 Voltage, 0.5V/div Voltage, 2V/div Figure 17(b). Start-up Transient (IOUT1 IOUT2 1.5A).
10ms/div Upper Trace OUT2 Voltage, 0.1V/div Middle Trace Voltage, 1V/div Lower Trace IL2, 2A/div
Figure 17(c). Channel Output Short-circuit Hiccup.
2008 Semtech Corp.
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SC2620
POWER MANAGEMENT Typical Application Circuits
BOOST1 COMP1 68pF 0.1µF 82.5k PGOOD1 0.1µF COMP2 47pF 12.4k 2.7nF BOOST2 1N4148 0.1µF 47µF 11.5k ROSC 1N4148 0.1µF 11.3k 2.2nF
60.4k 22µF 15.0k 47pF 47pF
OUT1 5V/2A
SC2620
PVIN
12-30V
UPS140
22µH
10µF 0.1µF UPS140
10µH 5.76k
OUT2 1.5V/2A
Murat GRM21BR60J226M Murat GRM31CR60J 476M C15: Murata GRM32DF51H106Z
Coiltronic DR74 Coiltronic DR73
Figure 18(a). 350kHz 12V-30V Input 1.5V Step-down Converter. Notice that Channel bootstrapped from OUT1. Channel will held OUT1 voltage below value.
Voltage, 10V/div Voltage, 10V/div
2µs/div Figure 18(b). Switching Waveforms. IOUT1= IOUT2=
Efficiency
Efficiency Efficiency Load Current OUT2 1.5V OUT1 Load Current VOUT2 1.5V VOUT1
Efficiency
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SC2620
POWER MANAGEMENT Outline Drawing SOIC-16
DIMENSIONS INCHES MILLIMETERS
.053 .069 .000 .005 .065 .049 .020 .012 .010 .007 .386 .390 .394 .150 .154 .157 .236 .050 .100 .105 .110 .080 .085 .090 .020 .010 .016 .028 .041 (.041) .004 .010 .008 1.75 0.13 1.65 0.51 0.25 9.90 10.00 3.90 4.00 6.00 1.27 2.54 2.67 2.79 2.03 2.16 2.29 0.50 0.25 0.40 0.72 1.04 (1.04) 0.10 0.25 0.20 1.35 0.00 1.25 0.31 0.17 9.80 3.80
TIPS
SEATING PLANE
EXPOSED
GAUGE PLANE 0.25 DETAIL (L1)
SIDE VIEW
NOTES:
DETAIL
CONTROLLING DIMENSIONS MILLIMETERS (ANGLES DEGREES). -B-HTO DETERMINED DATUM PLANE
DATUMS
DIMENSIONS "E1" INCLUDE MOLD FLASH, PROTRUSIONS GATE BURRS. REFERENCE JEDEC MS-012, VARIATION
Land Pattern SOIC-16
THERMAL 0.36mm SOLDER MASK
DIMENSIONS MILLIMETERS INCHES
(.205) .114 .201 .094 .118 .050 .024 .087 .291 (5.20) 2.90 5.10 2.40 3.00 1.27 0.60 2.20 7.40
NOTES: THIS LAND PATTERN REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES MET.
REFERENCE IPC-SM-782A, 300A. THERMAL VIAS LAND PATTERN EXPOSED SHALL CONNECTED SYSTEM GROUND PLANE. FAILURE COMPROMISE THERMAL AND/OR FUNCTIONAL PERFORMANCE DEVICE.
Contact Information
Semtech Corporation Power Management Products Division Flynn Road, Camarillo, 93012 Phone: (805)498-2111 (805)498-3804
2008 Semtech Corp.
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