Datasheet
1.24V
20 PA
LM3424
R
UV2
R
UV1
V
IN
UVLO
nDIM
R
UVH
(optional)
LM3424
OVP
R
OV2
R
OV1
LED+
LED-
2OVHYSO
RA20V xP=
¸
¸
¹
·
¨
¨
©
§
x
=
-OFFTURN
24V.1V
x
+
OV1
OV2
R5.0
R
1OV
R
1
¸
¸
¹
·
¨
¨
©
§
x
=
-OFFTURN
24V.1V
1OV
R
+
2OVOV
RR
LM3424
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SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
The LM3424 can be configured to detect an output (or input) over-voltage condition via the OVP pin. The pin
features a precision 1.24V threshold with 20 µA (typical) of hysteresis current as shown in Figure 31. When the
OVLO threshold is exceeded, the GATE pin is immediately pulled low and a 20 µA current source provides
hysteresis to the lower threshold of the OVLO hysteretic band.
If the LEDs are referenced to a potential other than ground (floating), as in the buck-boost and buck
configuration, the output voltage (V
O
) should be sensed and translated to ground by using a single PNP as
shown in Figure 32.
The over-voltage turn-off threshold (V
TURN-OFF
) is defined:
Ground Referenced
(29)
Floating
(30)
In the ground referenced configuration, the voltage across R
OV2
is V
O
- 1.24V whereas in the floating
configuration it is V
O
- 620 mV where 620 mV approximates V
BE
of the PNP.
The over-voltage hysteresis (V
HYSO
) is defined:
(31)
Figure 32. Floating Output OVP Circuitry
INPUT UNDER-VOLTAGE LOCKOUT (UVLO)
The nDIM pin is a dual-function input that features an accurate 1.24V threshold with programmable hysteresis as
shown in Figure 33. This pin functions as both the PWM dimming input for the LEDs and as a V
IN
UVLO. When
the pin voltage rises and exceeds the 1.24V threshold, 20 µA (typical) of current is driven out of the nDIM pin into
the resistor divider providing programmable hysteresis.
Figure 33. UVLO Circuit
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