LM3424 Datasheet, PDF (22/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
V_O
R_{OV2 OVP}
R_OV1
LM3424
20 2A
1.24V
OVLO
Figure 28. Over-Voltage Protection Circuitry
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R_OV2
LM3424
LED-
OVP
R_OV1
Figure 29. Floating Output OVP Circuitry
7.3.10 Input Undervoltage Lockout (UVLO)
The nDIM pin is a dual-function input that features an accurate 1.24V threshold with programmable hysteresis as
shown in Figure 30. 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.
V_IN
LM3424
20 2A
R_UV2
R_UVH
R_UV1
(optional)
nDIM
1.24V
UVLO
Figure 30. 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:
V_{TURN O-N} = 1.24V x _{¨¨©§}^RUV_R^{1 +}_UV^R₁ ^UV2_{¸¸¹·}
(31)
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