LM3424 Datasheet, PDF (28/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

15. SOFT-START
For all topologies, if soft-start is desired, find the start-up time
without C_SS (t_SU):
Then, if the desired total start-up time (t_TSU) is larger than
t_SU, solve for the base start-up time (t_SU-SS-BASE), assuming
that a C_SS greater than 40% of C_CMP will be used:
Then solve for C_SS:
16. PWM DIMMING METHOD
PWM dimming can be performed several ways:
Method #1: Connect the dimming MosFET (Q₃) with the drain
to the nDIM pin and the source to GND. Apply an external
PWM signal to the gate of Q_DIM. A pull down resistor may be
necessary to properly turn off Q₃.
Method #2: Connect the anode of a Schottky diode to the
nDIM pin. Apply an external inverted PWM signal to the cath-
ode of the same diode.
The DDRV pin should be connected to the gate of the dimFET
with or without level-shifting circuitry as described in the PWM
Dimming section. The dimFET should be rated to handle the
average LED current and the nominal output voltage.
17. ANALOG DIMMING METHOD
Analog dimming can be performed several ways:
Method #1: Place a potentiometer in place of the thermistor
in the thermal foldback circuit shown in the Thermal Folback /
Analog Dimming section.
Method #2: Place a potentiometer in series with the R_CSH
resistor to dim the LED current from the nominal I_LED to near
zero.
Method #3: Connect a controlled current source as detailed
in the Thermal Folback / Analog Dimming section to the CSH
pin. Increasing the current sourced into the CSH node will
decrease the LEDs from the nominal I_LED to zero current in
the same manner as the thermal foldback circuit.
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