LM3421 datasheet(38/72 Pages) TI1

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ID-MAX = ILED

ID-MAX = (1 - DMIN) x ILED

VRD-MAX = VIN-MAX + VO

VRD-MAX = VO

VRD-MAX = VIN-MAX

DSON

RMS

LED

T-

LED

RMS

IT-MAX =

x ILED

1 - DMAX

DMAX

IT-MAX = DMAX x ILED

MAX

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

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Typical Applications (continued)

(76)

Buck-Boost

(77)

The current rating should be at least 10% higher than the maximum average NFET current (IT-MAX):

Buck

(78)

Boost and Buck-Boost

(79)

Approximate the nominal RMS transistor current (IT-RMS) :

Buck

(80)

8.2.1.2.9.1

Boost and Buck-Boost

(81)

Given an NFET with on-resistance (RDS-ON), solve for the nominal power dissipation (PT):

(82)

8.2.1.2.10

Diode

The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (VRD-MAX):

Buck

(83)

Boost

(84)

Buck-Boost

(85)

The current rating should be at least 10% higher than the maximum average diode current (ID-MAX):

Buck

(86)

Boost and Buck-Boost

(87)

Replace DMAX with D in the ID-MAX equation to solve for the average diode current (ID). Given a diode with forward

voltage (VFD), solve for the nominal power dissipation (PD):

(88)

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LM3421 Datasheet(PDF) 38 Page - Texas Instruments

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