LM3421 datasheet(20/65 Pages) TI1

20 / 65 page

1.24V

23 PA

LM3421/23

RUV2

RUV1

VIN

UVLO

nDIM

RUVH

(optional)

LM3421/23

OVP

ROV2

ROV1

LED+

LED-

HYSO

- OFF

TURN

24V

OV1

OV2

LM3421, LM3421-Q1

LM3423, LM3423-Q1

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

www.ti.com

Floating

(27)

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

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

The over-voltage hysteresis (VHYSO) is defined:

(28)

Figure 27. Floating Output OVP Circuitry

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

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 VIN 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 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 (VTURN-ON) is defined as follows:

Submit Documentation Feedback

Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 20 Page - Texas Instruments

LM3421 Datasheet(HTML) 20 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Datasheet Download

Link URL