LM3421 datasheet(39/72 Pages) TI1

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(

)

23 2

(

)

HYS

UVH

UV1 =

1.24V

TURN

1.24V

UV2

UV2 =

23 2

HYS

OV1 =

1.24V

OV2

620

OFF

TURN

OV1 =

1.24V

OFF

TURN

1.24V

OV2

OV2 =

HYSO

23 2

LM3421, LM3421-Q1, LM3423, LM3423-Q1

www.ti.com

SNVS574F – JULY 2008 – REVISED JULY 2015

Typical Applications (continued)

8.2.1.2.11

Output OVLO

For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (VTURN-OFF)

and the desired hysteresis (VHYSO). To set VHYSO, solve for ROV2:

(89)

To set VTURN-OFF, solve for ROV1:

Boost

(90)

Buck-Boost

(91)

A small filter capacitor (COVP = 47 pF) should be added from the OVP pin to ground to reduce coupled switching

noise.

8.2.1.2.12

Input UVLO

For all topologies, input UVLO is programmed with the turnon threshold voltage (VTURN-ON) and the desired

hysteresis (VHYS).

Method 1: If no PWM dimming is required, a two resistor network can be used. To set VHYS, solve for RUV2:

(92)

To set VTURN-ON, solve for RUV1:

(93)

Method 2: If PWM dimming is required, a three resistor network is suggested. To set VTURN-ON, assume RUV2 =

10 k

Ω and solve for RUV1 as in Method 1. To set VHYS, solve for RUVH:

(94)

8.2.1.2.13

PWM Dimming Method

PWM dimming can be performed several ways:

Method 1: Connect the dimming MOSFET (Q3) with the drain to the nDIM pin and the source to AGND. Apply an

external PWM signal to the gate of QDIM. A pulldown resistor may be necessary to properly turn off Q3.

Method 2: Connect the anode of a Schottky diode to the nDIM pin. Apply an external inverted PWM signal to the

cathode of the same diode.

The DDRV pin should be connected to the gate of the dimFET with or without level-shifting circuitry as described

in the PWM Dimming section. The dimFET should be rated to handle the average LED current and the nominal

output voltage.

8.2.1.2.14

Analog Dimming Method

Analog dimming can be performed several ways:

Method 1: Place a potentiometer in series with the RCSH resistor to dim the LED current from the nominal ILED to

near zero.

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

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