Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB 데이터 시트
제품 코드
LM3431EVAL/NOPB
COMP
500 mV/Div
RFF
low
RFF
ideal
RFF
high
high
ILED
100 mA/Div
20 Ps/DIV
Vcathode
1V/Div
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
In PWM dimming, the average LED current is equal to the set LED current (I
LED
) multiplied by the dimming duty
cycle. The average LED current tracks the dimming ratio with exceptional linearity. However, the accuracy of
average LED current depends somewhat on the rise and fall times of the external current regulators. This
becomes more apparent with short on-times. To ensure good linearity, select NPN regulators with short and
similar rise and fall times.
average LED current depends somewhat on the rise and fall times of the external current regulators. This
becomes more apparent with short on-times. To ensure good linearity, select NPN regulators with short and
similar rise and fall times.
Setting FF
To minimize voltage transients during LED dimming, the output voltage is regulated via the AFB pin during LED
off times. However, because the control loop has a limited response time, voltage transients can never be
completely eliminated. If these transients are large enough, LED current will be affected and ceramic output
capacitors may generate audible noise. The FF pin speeds up the loop response time, and thus minimizes output
voltage transients during dimming.
off times. However, because the control loop has a limited response time, voltage transients can never be
completely eliminated. If these transients are large enough, LED current will be affected and ceramic output
capacitors may generate audible noise. The FF pin speeds up the loop response time, and thus minimizes output
voltage transients during dimming.
A resistor connected from FF to ground, Rff, sets the FF current which is injected into the control loop at the
rising and falling edge of the dimming signal. In this way, the FF pin creates a correction signal before the control
loop can respond. A smaller FF resistor will generate a larger correction signal. The minimum recommended Rff
value is 10k.
rising and falling edge of the dimming signal. In this way, the FF pin creates a correction signal before the control
loop can respond. A smaller FF resistor will generate a larger correction signal. The minimum recommended Rff
value is 10k.
Since the amount of FF correction required for a given application depends on many factors, it is best to
determine a FF resistor value through bench testing. Use the following procedure to determine an optimal Rff
value:
determine a FF resistor value through bench testing. Use the following procedure to determine an optimal Rff
value:
An Rff value of approximately 20k is a good starting point. A 20 k
Ω
potentiometer in series with a 10 k
Ω
resistor
works well for bench testing.
The dimming frequency must be selected before setting Rff. Confirm that boost switching operation is stable at
100% dimming duty cycle.
100% dimming duty cycle.
Adjust Rff until the COMP pin voltage is between 0.8V and 0.9V. Next, monitor the cathode voltage response at
a low dimming duty cycle while adjusting Rff until the overshoot and undershoot is minimal or there is a slight
overshoot.
a low dimming duty cycle while adjusting Rff until the overshoot and undershoot is minimal or there is a slight
overshoot.
Check the cathode voltage response at the lowest input voltage and lowest dimming duty cycle and adjust Rff if
necessary. This is typically the worst case condition.
necessary. This is typically the worst case condition.
The curves in
below show the variation in cathode voltage with different Rff settings. Notice that at the
ideal setting, both the cathode voltage and COMP voltage are flat. For clarity, the 3 cathode voltage curves in
this figure have been offset; all FF settings will result in the cathode voltage settling at 1.2V typically.
this figure have been offset; all FF settings will result in the cathode voltage settling at 1.2V typically.
Figure 23. FF Setting Example
Once an Rff value has been set, check the cathode voltage over the input voltage range and dimming duty
range. Some further adjustment may be necessary.
range. Some further adjustment may be necessary.
Copyright © 2007–2013, Texas Instruments Incorporated
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