LM3421_09 Datasheet, PDF (8/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

30kHz PWM Dimming
(95% Duty Cycle ON)
30kHz PWM Dimming
(10% Duty Cycle ON)
I_LED = 1A nominal, V_IN = 12V, V_LED(s) = 20V
Top trace: nDIM input, 2V/div, DC
Bottom trace: I_LED, 500mA/div, DC
T = 10µs/div
30kHz PWM Dimming
(5% Duty Cycle ON)
30067369
I_LED = 1A nominal, V_IN = 12V, V_LED(s) = 20V
Top trace: nDIM input, 2V/div, DC
Bottom trace: I_LED, 500mA/div, DC
T = 10µs/div
30067370
I_LED = 1A nominal, V_IN = 12V, V_LED(s) = 20V
Top trace: nDIM input, 2V/div, DC
30067371
Bottom trace: I_LED, 500mA/div, DC
T = 10µs/div
As shown in the Average LED Current vs. PWM DIM Duty Cycle curve the current
drops at very low duty cycle. This is due to the fact that the switcher is shut off
during the LED off time. At low duty cycle the choke does not have time to get
the current up to the control setpoint. This can be looked on as an advantage in
that it allows a wider PWM control range, albeit not linear under about 7.5% duty
cycle (this transition point can be set over a limited range by component value
selection).
www.national.com
8