LM3424 Datasheet, PDF (24/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
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Feature Description (continued)
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_IN²
(34)
Even maintaining a dimming pulse greater than t_PULSE, preserving linearity at low dimming duty cycles is difficult.
Several modifications are suggested for applications requiring low dimming duty cycles. Since nDIM rising
releases the latch but does not trigger the on-time specifically, there will be an effective jitter on the rising edge of
the LED current. This jitter can be easily removed by tying the PWM input signal through the synchronization
network at the RT pin (shown in Figure 35), forcing the on-time to synchronize with the nDIM pulse.
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.
LM3424
5 k:
10:
Q7 100 nF
LED+
V_CC
DDRV
Q6
10V
500:
Q4
100 pF
Q2
R_SNS
V_IN
Figure 32. Buck-boost Level-Shifted PWM Circuit
LM3424
100
k:
100 nF
DDRV
R_SNS
10V
Q2
Figure 33. Buck Level-Shifted PWM Circuit
24
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