LM3424 Datasheet, PDF (23/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
The hysteresis (V_HYS) is defined as follows:
7.3.10.1 UVLO Only
V_HYS = 20 2A x R_UV2
(32)
7.3.10.2 PWM Dimming and UVLO
V_HYS
=
20
PA
x
¨¨©§^RUV2
+
R_UVH
x (R_UV1 +
R_UV1
R_UV2)_{¸¸¹·}
(33)
7.3.11 PWM Dimming
The active low nDIM pin can be driven with a PWM signal which controls the main NFET and the dimming FET
(dimFET). The brightness of the LEDs can be varied by modulating the duty cycle of this signal. LED brightness
is approximately proportional to the PWM signal duty cycle, (that is, 30% duty cycle at approximately 30% LED
brightness). This function can be ignored if PWM dimming is not required by using nDIM solely as a V_IN UVLO
input as described in the Input Undervoltage Lockout (UVLO) section or by tying it directly to V_CC or V_IN.
Inverted
V_IN
PWM
LM3424
R_UV2
R_UVH
R_UV1
D_DIM
nDIM
Q_DIM
Standard
PWM
Figure 31. PWM Dimming Circuit
Figure 31 shows how the PWM signal is applied to nDIM:
1. Connect the dimming MOSFET (Q_DIM) with the drain to the nDIM pin and the source to GND. Apply an
external logic-level PWM signal to the gate of Q_DIM.
2. Connect the anode of a Schottky diode (D_DIM) to the nDIM pin. Apply an inverted external logic-level PWM
signal to the cathode of the same diode.
The DDRV pin is a PWM output that follows the nDIM PWM input signal. When the nDIM pin rises, the DDRV pin
rises and the PWM latch reset signal is removed allowing the main MOSFET Q1 to turn on at the beginning of
the next clock set pulse. In boost and buck-boost topologies, the DDRV pin is used to control a N-channel
MOSFET placed in series with the LED load, while it would control a P-channel MOSFET in parallel with the load
for a buck topology.
The series dimFET will open the LED load, when nDIM is low, effectively speeding up the rise and fall times of
the LED current. Without any dimFET, the rise and fall times are limited by the inductor slew rate and dimming
frequencies above 1 kHz are impractical. Using the series dimFET, dimming frequencies up to 30 kHz are
achievable. With a parallel dimFET (buck topology), even higher dimming frequencies are achievable.
When using the PWM functionality in a boost regulator, the PWM signal drives a ground referenced FET.
However, with buck-boost and buck topologies, level shifting circuitry is necessary to translate the PWM dim
signal to the floating dimFET as shown in Figure 32 and Figure 33.
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