2003 APPLICATION NOTE 2198 Driving LEDs with Open Drain Port
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Maxim Notes MICROCONTROLLERS Keywords: driver, driving, open drain, GPIOs, port expander, LEDs, expanders
2003
APPLICATION NOTE 2198
Driving LEDs with Open Drain Port Expander Outputs
Abstract: This application note discusses techniques driving LEDs with MAX6964, MAX6965, MAX7313, MAX7314, MAX7315 MAX7316 expanders (GPIOs). These techniques applied other expanders with open-drain outputs, well other with open-drain logic outputs. (Types like 74HC06 74HC07 used high current drive applications).
Basics
standard connection driving from port shown Figure load typically single LED, dual LEDs series shown), depending choice LEDs supply voltage. series resistor necessary limit current through LED. value resistor required drive current IPORT through calculated using formula: (VEXT VPORT VLED) IPORT where VEXT supply voltage VLED voltage drop across LED(s) required load current (usually range 1.8V 2.4V LEDs, 4.2V blue, white, high efficiency green LEDs) VPORT voltage drop across output port when sinking required load current (for example 0.25V 20mA MAX6964)
Figure Standard connection.
VEXT does have same voltage port expander supply pin. protection within many (but all!) IC's open-drain output structures allow load connected voltage above chip supply voltage (but negative with respect GND). example, MAX6964, MAX6965 outputs rated allowing LEDs Figure circuit connected instead 3.3V port expander supply. higher supply voltage allows LEDs series alternatively single white LED) driven. current will vary with supply voltage forward voltage. Resistor initial accuracy temperature coefficient, plus variance port output voltage with temperature supply voltage, will also play part. consistent current important, make sure that voltage drop across resistor high compared with total voltage variances VEXT supply LED. example, consider situation where need drive nominal 20mA through whose forward voltage ±0.2V. have choice either 3.3V VEXT. using MAX6964 port which will drop 0.25V ±0.1V when sinking 20mA. Using typical values, value calculates 0.2) 0.02 supply case, (3.3 0.2) /0.02 3.3V case. Using these exact values then actual current variation tolerance extremes would ±3.9mA 19.5% variation) case, ±8.5mA ±42% variation) 3.3V case. Clearly 3.3V solution much wider current variation than solution. However, solution dissipates more power current limiting resistor. this current variation high, consider using port expander with constant-current (internally current limited) outputs such MAX6956 MAX6957, techniques such shown Figures which external transistor control current more accurately.
Driving LEDs Parallel
Multiple LEDs driven parallel share current from single output (Figure Series resistors current through each LED. Driving LEDs parallel opposed series reduces drive voltage headroom required, also reduces maximum drive current available each LED.
Figure Driving multiple LEDs from output.
Want More Current? More Ports
more load current required than available from port, multiple ports paralleled wire-OR fashion drive single load. Because outputs open drain, multiple outputs opposite levels won't short. However open drain outputs current limited, they need prevented from carrying excessive current. safe avoiding overloading separate current limiting resistor port (Figure When both outputs low, resistors current safe level each output. ports switched opposite levels, half current will flow, this scheme provides some current intensity control (off current full current).
Figure Paralleling otputs safe way. alternative approach guarantee that paralleled ports switched together. Many port expanders (MAX6964, MAX6965, MAX7313, MAX7314, MAX7315 MAX7316 included) register structure that allows multiple outputs switched simultaneously with same software command. this case, becomes software issue ensure that ports will always same level, circuit Figure used. Remember that programming ports opposite logic levels will mean that output will sink full current expected shared between outputs.
Figure Paralleling outputs lower cost way.
Even More Drive Voltage Current? Transistor
discussed earlier, output loads must connected voltage outside port expander's rated range. This becomes problem when driving many series LEDs high current needed. circuits shown Figures external transistor extend voltage drive and/or drive current. first circuit uses single transistor switch which essentially replaces port expander's output driver (Figure transistor ratings alone determine much current voltage handled. Advantages this circuit compared with circuits discussed next wasted voltage across because turned hard, current will vary with port expander supply voltage. Disadvantages that current will vary with VLED supply voltage, that circuit uses resistors.
Figure Driving LEDs with higher current from higher positive voltage. circuit Figure allows positive voltage used VEXT. circuit Figure allows negative voltage used, advantage that current flows from port expander supply negative supply, making supplies available drive LEDs. optional resistor ensures that transistor when port output high (high impedance), should only necessary elevated temperatures when becomes relatively leaky. Note that Figure circuit doesn't draw current when off, unlike Figure circuit which always draws current through
Figure Driving LEDs with higher current from negative voltage.
What about Constant Current?
drive current circuits Figure varies with supply voltage, VEXT. VEXT well regulated, this could problem. circuit Figure operates common-base (cascode) current switch. port output VPORT constrained safe voltage VBE) because Q3's base tied driver supply voltage, Q2's emitter current IPORT calculated using formula: IPORT VPORT VBE) Amps port current Iport (also Q3's emitter current) will flow through Q3's collector loads, less small base current taken current error Q3's base current kept below choosing reasonably high gain 100) transistor supply voltage doesn't vary much (VPORT won't), then this circuit works pretty good constant current sink, dependent port expander supply voltage, independent supply voltage VLED.
Figure Active-Low, constant current sink drive. disadvantage Figure circuit that load current flows through port expander active output, thereby limiting maximum current that port expander. Figure circuit avoids this problem, allows current well voltage rating determined transistor ratings. Q3's emitter current ILED calculated using formula: ILED VBE) Amps Figure circuit port expander active high output supplying Q3's base current only, which small enough that port output voltage drop negligible ignored. Figure shows mirror topology Figure with pass transistor replacing NPN. LEDs referred negative supply instead higher voltage positive supply. current supplied from port expander's positive supply returned into negative supply. Figure circuit, optional resistor ensures that transistor when port output high (high impedance), should only necessary elevated temperatures when becomes relatively leaky.
Figure Active-Low, constant current source drive. limitation Figure circuit that collector external transistor lower than VCE(sat)), which only little below high, example then only output expander dissipating high power this high voltage drop forced across output, also voltage headroom available LEDs reduced almost (VLED V+). simple solution bias Q3's base lower voltage using series zener diode silicon diode, depending voltage drop needed. Only diode needed serve multiple pass transistors, shown example circuit Figure With this circuit Q5's emitter voltage sits about when supply voltage 3.3V.
Figure Using Zener Diode Minimize Driver Headroom.
Also:
Driving LEDs with Constant Current Port Expander Outputs Driving LEDs with Push-Pull Port Expander Outputs
Application Note 2198: http://www.maxim-ic.com/an2198 More Information technical questions support: http://www.maxim-ic.com/support samples: http://www.maxim-ic.com/samples Other questions comments: http://www.maxim-ic.com/contact Related Parts MAX6964: QuickView Full (PDF) Data Sheet Free Samples MAX6964: QuickView Full (PDF) Data Sheet Free Samples MAX6965: QuickView Full (PDF) Data Sheet Free Samples MAX6965: QuickView Full (PDF) Data Sheet Free Samples MAX7313: QuickView Full (PDF) Data Sheet Free Samples MAX7313: QuickView Full (PDF) Data Sheet Free Samples MAX7314: QuickView Full (PDF) Data Sheet Free Samples MAX7315: QuickView Full (PDF) Data Sheet Free Samples MAX7315: QuickView Full (PDF) Data Sheet Free Samples MAX7316: QuickView Full (PDF) Data Sheet Free Samples MAX7316: QuickView Full (PDF) Data Sheet Free Samples MAX7317: QuickView Full (PDF) Data Sheet Free Samples MAX7317: QuickView Full (PDF) Data Sheet Free Samples
AN2198, 2198, APP2198, Appnote2198, Appnote 2198 Copyright Maxim Integrated Products Additional legal notices: http://www.maxim-ic.com/legal
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