LM3421_09 Datasheet, PDF (17/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

Another method of limiting the turn on overshoot is the selec-
tion of the R_IS Resistor that is between the IS pin and PGND.
If the MOSFET channel resistance is used there is little that
can be done, but by using a separate sense resistor placed
in the source of the Mosfet (Please refer to Figure 13 for circuit
details) the peak input current can be set. Setting the peak
current at the peak input voltage sets the peak power inter-
cepted from the input during turn on. This sets the rate of rise
of voltage on the output capacitor, and consequently the
amount of overshoot seen as the control loop settles to its
programmed value.
SLOW SHUTDOWN FEATURE (FADE OUT)
In some applications, particularly automotive interior lighting,
it may be desirable for the LEDs to transition slowly to the off
state rather than abruptly shutting off. This can be easily ac-
complished with a few small and inexpensive external com-
ponents, as show in Figure 9.
This circuit simply delays the shutdown on the EN pin and
slowly decreases the amount of current in the LEDs by de-
creasing the amount of current flowing out of the CSH pin,
which is directly proportional to the LED current as previously
discussed in the section titled PROGRAMMING AVERAGE
LED CURRENT.
DESIGN EXAMPLES
The following set of schematics show the LM3421/LM3423 in
various topology and feature set combinations. For more
complete schematics and associated Bills of Materials
(BoMs) and circuit board layouts, please see the application
notes associated with the various demonstration boards that
are available for these products.
30067361
FIGURE 9. LM3421/LM3423 Slow Shutdown Circuit
FIGURE 10. LM3421 Boost Topology with High Speed Dimming
30067331
17
www.national.com