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SNVS574E – JULY 2008 – REVISED MAY 2013
The series dimFET will open the LED load, when nDIM is low, effectively speeding up the rise and fall times of
the LED current. Without any dimFET, the rise and fall times are limited by the inductor slew rate and dimming
frequencies above 1 kHz are impractical. Using the series dimFET, dimming frequencies up to 30 kHz are
achievable. With a parallel dimFET (buck topology), even higher dimming frequencies are achievable.
When using the PWM functionality in a boost regulator, the PWM signal can drive a ground referenced FET.
However, with buck-boost and buck topologies, level shifting circuitry is necessary to translate the PWM dim
signal to the floating dimFET as shown in Figure 30 and Figure 31 . If high side dimming is necessary in a boost
regulator using the LM3423, level shifting can be added providing the polarity inverting DPOL pin is pulled low
When using a series dimFET to PWM dim the LED current, more output capacitance is always better. A general
rule of thumb is to use a minimum of 40 μF when PWM dimming. For most applications, this will provide
adequate energy storage at the output when the dimFET turns off and opens the LED load. Then when the
dimFET is turned back on, the capacitance helps source current into the load, improving the LED current rise
time.
A minimum on-time must be maintained in order for PWM dimming to operate in the linear region of its transfer
function. Because the controller is disabled during dimming, the PWM pulse must be long enough such that the
energy intercepted from the input is greater than or equal to the energy being put into the LEDs. For boost and
buck-boost regulators, the minimum dimming pulse length in seconds (t PULSE ) is:
2 x I LED x V O X L1
t PULSE =
V IN 2
(32)
Even maintaining a dimming pulse greater than t PULSE , preserving linearity at low dimming duty cycles is difficult.
The second helpful modification is to remove the C FS capacitor and R FS resistor, eliminating the high frequency
compensation pole. This should not affect stability, but it will speed up the response of the CSH pin, specifically
at the rising edge of the LED current when PWM dimming, thus improving the achievable linearity at low dimming
duty cycles.
LED+
LM3421/23
10 :
5 k :
Q7
100 nF
Q2
V CC
Q6
Q4
R SNS
10V
500 :
100 pF
V IN
DDRV
Figure 30. Buck-boost Level-Shifted PWM Circuit
22
Copyright ? 2008–2013, Texas Instruments Incorporated
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