Datasheet

LM3429
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
1.24V
20 PA
LM3429
R
UV2
R
UV1
V
IN
UVLO
nDIM
R
UVH
(optional)
LM3429, LM3429-Q1
SNVS616G –APRIL 2009–REVISED MAY 2013 www.ti.com
Figure 25. UVLO Circuit
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 necessary when PWM dimming if operating near the UVLO threshold.
The turn-on threshold (V
TURN-ON
) is defined as follows:
(23)
The hysteresis (V
HYS
) is defined as follows:
UVLO only
(24)
PWM dimming and UVLO
(25)
PWM DIMMING
The active low nDIM pin can be driven with a PWM signal which controls the main NFET (Q1). 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, so 30% duty cycle equals 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
UNDER-VOLTAGE LOCKOUT (UVLO) section or by tying it directly to V
CC
or V
IN
(if less than 76VDC).
Figure 26. PWM Dimming Circuit
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