LM3429_14 Datasheet, PDF (17/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

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LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
LED+
R_OV2
LM3429
LED-
OVP
R_OV1
Figure 12. Floating Output OVP Circuitry
The over-voltage turn-off threshold (V_TURN-OFF) is defined as follows:
Ground Referenced
V_{TURN- OFF} = 1.24V x _{¨¨©§}^ROV_R^{1 +}_OV^R₁ ^OV2_{¸¸¹·}
(20)
Floating
V_{TURN -}
OFF
=
1.24V
x
¨¨©§^0.5
x
R_OV1+
R_OV1
R _{OV2¸¸¹·}
(21)
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 the V_BE of the PNP transistor.
The over-voltage hysteresis (V_HYSO) is defined as follows:
V_HYSO = 20 PA x R_OV2
(22)
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 13. 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
LM3429
20 PA
R_UV2
R_UVH
R_UV1
(optional)
nDIM
1.24V
UVLO
Figure 13. 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:
V_{TURN O-N} = 1.24V x _{¨¨©§}^RUV_R^{1 +}_UV^R₁ ^UV2_{¸¸¹·}
(23)
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