TPS61090

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SLVA211 June 2005
Powering White Flashlight With TPS61090
Steven Power Management Products/Portable Power DC-DC Applications
ABSTRACT This application report details drive white from single lithium-ion battery using Texas Instruments TPS61090 low-power dc-dc boost converter achieve efficiencies >90%. Design considerations equations needed driving single, multiple-watt white covered including efficiency improvements, peak switch current calculations, soft-start considerations.
High-power LEDs becoming popular portable handheld equipment. Because multiple-watt LEDs have forward voltage drops order boost converter necessary when powering from single lithium-ion battery (2.7 maintain constant current. Texas Instruments TPS61xxx boost converter family offers wide range flexibility this application. regulated current source drives white LEDs monitoring voltage drop across current sense resistor. General information regarding white drivers provided TPS61040EVM-002 User's Guide (SLVU068). When driving single, multiple-watt LED, three additional factors should considered: Increasing efficiency injecting voltage into feedback Staying under peak switch current limit internal Maintaining start-up conditions necessary correct soft-start operation following paragraphs explain design single using TPS61090 synchronous boost converter with switch. Table General Design Parameters
Design Requirements Iout Target Efficiency V(sense resistor) V(cs) 0.23 Boost Converter TPS61090 Texas Instruments F(sw) typical, minimum (switch current limit) typical, minimum Part Requirements W5SG Osram Opto 3.50
SLVA211 June 2005
Powering White Flashlight With TPS61090
www.ti.com
Figure Schematic White Driver Using TPS61090
Powering White Flashlight With TPS61090
SLVA211 June 2005
www.ti.com
Increasing efficiency injecting voltage into feedback Minimizing power dissipation current sense resistor increases efficiency thus lengthens battery life. application report Extending Battery Life with TPS61040 White Light Driver (SLEA004) explains inject voltage into feedback integrated circuit (IC) under no-load conditions reduce voltage (and thus power dissipation) across current sense resistor. Sizing resistors sets appropriate voltages feedback across sense resistor. reference voltage (Vref) TPS61090, voltage current sense node, VCS, 0.23 Figure shows that this method improves efficiency (LED Power/Input Power) average
Vref
Where forward voltage across LED, desired voltage across current sense resistor full current, Vref feedback voltage TPS61090 this design, chosen yielding equivalent resistance from cathode ground determines current. chosen thus negligible effect current. used start-up described later this application report. Rdson Iout Where desired voltage across current sense resistor full current, Iout full current, Rdson drain-to-source resistance N-channel chosen have Rdson approximately yielding
0.95 Inproved Efficiency, 0.239 Efficiency 0.90 Normal Efficiency, 0.498 0.85
0.80
0.75
2.75
3.25
3.75
4.25
Input Voltage
Figure TPS61090 White Driver Efficiency Input Voltage Staying under peak switch current limit internal TPS61xxx family dc-dc boost converters integrated FETs increase ease while decreasing component count necessary board space. Care must taken exceed peak switch current limit internal FETs. boost converter, direct-current component depends current alternating-current component depends input output voltages, switching frequency, inductance.
SLVA211 June 2005
Powering White Flashlight With TPS61090
www.ti.com
Iripple_ppk Isw_max Vout Iout Vin_min Vcs) Iout Isw_max Vin_min Vi_min Lmin Fmin Vin_min
Where desired voltage across current sense resistor full current, forward voltage across LED, Iout full current, target power supply efficiency, Vin_min minimum input voltage, Lmin minimum inductance, Fmin minimum power supply switching frequency conservative estimate converter efficiency recommended inductor reasonable values used above calculations. calculated peak switch current must less than 2000 TPS61090; this design, 1.316 0.103 1.419 Maintaining start-up conditions necessary correct soft-start sequencing TPS61090 undergoes multiple discrete soft-start stages initial turnon requires minimum equivalent load resistance start properly. minimum load needed highest input voltages; this design, minimum load required This achieved adding N-channel series with current sense resistor. When part initially enabled, high impedance, current sense resistance charges, turns current sense resistor becomes parallel with (0.3 Transitioning current sense resistor between effectively soft-starts current. this design, were chosen respectively, give start-up circuit 1-ms time constant. chosen minimize excess power dissipation during start-up. References Extending Battery Life With TPS61040 White Light Driver application report (SLEA004) TPS61090, TPS61091, TPS61092, Synchronous Boost Converter with Switch data sheet (SLVS484) TPS61040EVM-002 White Light Bias Supply Evaluation Module User's Guide (SLVU068)
Powering White Flashlight With TPS61090
SLVA211 June 2005
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TPS61090

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