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V TURN - OFF = 1 . 24V x ¨ ¨
R OV 1
SNVS574E – JULY 2008 – REVISED MAY 2013
Floating
?
§ 0 . 5 x R OV1 + R OV2 ·
?
? 1
(27)
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:
V HYSO = 23 P A x R OV 2
(28)
LED+
R OV2
LM3421/23
LED-
OVP
R OV1
Figure 27. Floating Output OVP Circuitry
The OVLO feature can cause some interesting results if the OVLO trip-point is set too cose to V O . At turn-on, the
converter has a modest amount of voltage overshoot before the control loop gains control of I LED . If the overshoot
exceeds the OVLO threshold, the controller shuts down, opening the dimming MosFET. This isolates the LED
load from the converter and the output capacitance. The voltage will then discharge very slowly through the HSP
and HSN pins until V O drops below the lower threshold, where the process repeats. This looks like the LEDs are
blinking at around 2 Hz. This mode can be escaped if the input voltage is reduced.
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 28 . 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, 23 μA (typical) of current is driven out of the nDIM pin into
the resistor divider providing programmable hysteresis.
V IN
LM3421/23
23 P A
R UV2
R UV1
R UVH
(optional)
nDIM
1.24V
UVLO
Figure 28. 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 preferable when PWM dimming, if operating near the UVLO threshold.
The turn-on threshold (V TURN-ON ) is defined as follows:
20
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