Datasheet

LM3424
R
UV2
R
UV1
V
IN
nDIM
R
UVH
Q
DIM
D
DIM
Inverted
PWM
Standard
PWM
x
PA20
=
HYS
V
¨
¨
©
§
+
2UV
R
¸
¸
¹
·
1UV
R
(
+
x
1UV
R
)
2UV
R
UVH
R
2UV
RA20
x
P
HYS
V
=
1
¸
¸
¹
·
¨
¨
©
§
x
=
-ONTURN
24V.1V
1UV
R
+
2UVUV
RR
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
When using the nDIM pin for UVLO and PWM dimming concurrently, the UVLO circuit can have an extra series
resistor to set the hysteresis. This allows the standard resistor divider to have smaller resistor values minimizing
PWM delays due to a pull-down MosFET at the nDIM pin (see PWM DIMMING section). In general, at least 3V
of hysteresis is preferable when PWM dimming, if operating near the UVLO threshold.
The turn-on threshold (V
TURN-ON
) is defined as follows:
(32)
The hysteresis (V
HYS
) is defined as follows:
UVLO only
(33)
PWM dimming and UVLO
(34)
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 V
IN
UVLO input as described
in the INPUT UNDER-VOLTAGE LOCKOUT (UVLO) section or by tying it directly to V
CC
or V
IN
.
Figure 34. PWM Dimming Circuit
Figure 34 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.
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