LM3421 datasheet(20/72 Pages) TI1

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LM3421/23

OVP

ROV2

ROV1

LED+

LED-

1.24V

23 2A

LM3421/23

ROV2

ROV1

OVLO

OVP

HYSO

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

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Feature Description (continued)

In the ground referenced configuration, the voltage across ROV2 is VO – 1.24 V whereas in the floating

configuration it is VO – 620 mV where 620 mV approximates VBE of the PNP.

The overvoltage hysteresis (VHYSO) is defined using Equation 25.

(25)

Figure 21. Overvoltage Protection Circuitry

Figure 22. Floating Output OVP Circuitry

The OVLO feature can cause some interesting results if the OVLO trip-point is set too close to VO. At turnon, the

converter has a modest amount of voltage overshoot before the control loop gains control of ILED. 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 VO 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.

7.3.11 Input Undervoltage Lockout (UVLO)

The nDIM pin is a dual-function input that features an accurate 1.24-V threshold with programmable hysteresis

as shown in Figure 23. This pin functions as both the PWM dimming input for the LEDs and as a VIN UVLO.

When the pin voltage rises and exceeds the 1.24-V threshold, 23 µA (typical) of current is driven out of the nDIM

pin into the resistor divider providing programmable hysteresis.

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LM3421 Datasheet(PDF) 20 Page - Texas Instruments