LM3421 datasheet(21/65 Pages) TI1

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LM3421/23

RUV2

RUV1

VIN

nDIM

RUVH

QDIM

DDIM

Inverted

PWM

Standard

PWM

HYS

¨¨

¸¸

(

)

UVH

HYS

TURN

24V

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

(29)

The hysteresis (VHYS) is defined as follows:

UVLO only

(30)

PWM dimming and UVLO

(31)

When "zero current" shutdown and UVLO are implemented together, the EN pin can be used to escape UVLO.

The nDIM pin will pull-up to VIN when EN is pulled low, therefore if VIN is within the UVLO hysteretic window

when EN is pulled high again, the controller will start-up even though VTURN-ON is not exceeded.

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, (i.e. 30% duty cycle ~ 30% LED brightness). This

function can be ignored if PWM dimming is not required by using nDIM solely as a VIN UVLO input as described

in the INPUT UNDER-VOLTAGE LOCKOUT (UVLO) section or by tying it directly to VCC or VIN.

Figure 29. PWM Dimming Circuit

Figure 29 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.

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