LM3421 datasheet(17/24 Pages) NSC

17 / 24 page

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.

30067361

FIGURE 9. LM3421/LM3423 Slow Shutdown Circuit

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.

30067331

FIGURE 10. LM3421 Boost Topology with High Speed Dimming

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