LM3421 datasheet(35/72 Pages) TI1

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P =

r x

1 Z

LIM

245 mV

1-DMAX

DMAX

ICO-RMS = ILED x

RMS

LED

LED PP

C =

LED

LM3421, LM3421-Q1, LM3423, LM3423-Q1

www.ti.com

SNVS574F – JULY 2008 – REVISED JULY 2015

Typical Applications (continued)

8.2.1.2.5

LED Ripple Current

Set the nominal LED ripple current (

ΔiLED-PP), by solving for the output capacitance (CO):

Buck

(48)

Boost and Buck-boost

(49)

To set the worst case LED ripple current, use DMAX when solving for CO. Remember, when PWM dimming, TI

recommends using a minimum of 40 µF of output capacitance to improve performance.

The minimum allowable RMS output capacitor current rating (ICO-RMS) can be approximated:

Buck

(50)

Boost and Buck-boost

(51)

8.2.1.2.6

Peak Current Limit

Set the peak current limit (ILIM) by solving for the transistor path sense resistor (RLIM):

(52)

8.2.1.2.7

Loop Compensation

Using a simple first order peak current mode control model, neglecting any output capacitor ESR dynamics, the

necessary loop compensation can be determined.

First, the uncompensated loop gain (TU) of the regulator can be approximated:

Buck

(53)

Boost and Buck-Boost

(54)

Where the pole (

ωP1) is approximated:

Buck

(55)

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LM3421 Datasheet(PDF) 35 Page - Texas Instruments

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