LM3421_10 Datasheet, PDF (16/48 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

OVER-VOLTAGE LOCKOUT (OVLO)
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FIGURE 12. Over-Voltage Protection Circuitry
The LM3421/23 can be configured to detect an output (or in-
put) over-voltage condition via the OVP pin. The pin features
a precision 1.24V threshold with 23 µA (typical) of hysteresis
current as shown in Figure 12. When the OVLO threshold is
exceeded, the GATE pin is immediately pulled low and a 23
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 13.
The over-voltage turn-off threshold (V_TURN-OFF) is defined:
Ground Referenced
Floating
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:
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 14. 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.
FIGURE 14. UVLO Circuit
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When using the nDIM pin for UVLO and PWM dimming con-
currently, 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:
The hysteresis (V_HYS) is defined as follows:
UVLO only
PWM dimming and UVLO
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FIGURE 13. Floating Output OVP Circuitry
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 V_IN when EN is pulled low, therefore if V_IN is
within the UVLO hysteretic window when EN is pulled high
again, the controller will start-up even though V_TURN-ON is not
exceeded.
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