LM3421_09 Datasheet, PDF (16/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

bring the current down below the 30% high threshold and the
fault timer interval. For short timer intervals, the controller
simply shuts off in the fault lockdown mode. The user will ob-
serve the LEDs blink on once at power up should this condi-
tion exist. This overshoot of current can be prevented by
adding a control zero into the system as detailed in Figure 7.
Simply adding the shaded components eliminates this issue
(see Figure 8). Note that for many configurations these com-
ponents will not be required.
FIGURE 7. Boost LED Driver with Lead Compensation
30067305
30067372
I_LED = 1A nominal, V_IN = 12V, V_LED(s) = 20V, no lead compensation
Top trace: EN input, 1V/div, DC
Bottom trace: I_LED, 500mA/div, DC
T = 1ms/div
30067373
I_LED = 1A nominal, V_IN = 12V, V_LED(s) = 20V, with lead compensation
Top trace: EN input, 1V/div, DC
Bottom trace: I_LED, 500mA/div, DC
T = 1ms/div
FIGURE 8. Scope Plots for Startup With and Without Lead Compensation
www.national.com
16