LM3421 datasheet(22/72 Pages) TI1

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tPULSE =

2 x ILED x VO X L1

VIN

LM3421/23

RUV2

RUV1

VIN

nDIM

RUVH

QDIM

DDIM

Inverted

PWM

Standard

PWM

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

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Feature Description (continued)

Figure 24. PWM Dimming Circuit

Figure 24 shows how the PWM signal is applied to nDIM:

1. Connect the dimming MOSFET (QDIM) with the drain to the nDIM pin and the source to AGND. Apply an

external logic-level PWM signal to the gate of QDIM.

2. Connect the anode of a Schottky diode (DDIM) 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 can drive 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 25 and Figure 26. If high side dimming is necessary in a boost

regulator using the LM3423, level shifting can be added providing the polarity inverting DPOL pin is pulled low

(see LM3423 Only: DPOL, FLT, TIMR, and LRDY section) as shown in Figure 27.

When using a series dimFET to PWM dim the LED current, more output capacitance is always better. A general

rule of thumb is to use a minimum of 40 µF when PWM dimming. For most applications, this will provide

adequate energy storage at the output when the dimFET turns off and opens the LED load. Then when the

dimFET is turned back on, the capacitance helps source current into the load, improving the LED current rise

time.

A minimum on-time must be maintained in order for PWM dimming to operate in the linear region of its transfer

function. Because the controller is disabled during dimming, the PWM pulse must be long enough such that the

energy intercepted from the input is greater than or equal to the energy being put into the LEDs. For boost and

buck-boost regulators, the minimum dimming pulse length in seconds (tPULSE) is:

(29)

Even maintaining a dimming pulse greater than tPULSE, preserving linearity at low dimming duty cycles is difficult.

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LM3421 Datasheet(PDF) 22 Page - Texas Instruments

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