D = 1 -

'¶ = 1-D =

LM3431

VIN

EN

FF

SGND

AFB

VCC

REF

REFIN

SNS1

NDRV3

SNS3

NDRV1

NDRV2

SNS2

LG

CS

PGND

COMP

MODE/F

RT

CFB

THM

SC

SS/SH

DLY

DIM

ILIM

Vin

LEDOFF

REFIN

LEDOFF

VCC

D2-5

D6-9

D1

L1

Q1

C9

R19

R18

R3

R4

C1

C2

R2

R1

R7

R8

R9

C4

Rff

C3

R6

C6

C7

VCC

R10

Q2

Q6

External LED Array

R15

VA

VC1

VC2

VC3

VC4

RMODE

C5

THM

R17

VCC

Op1

THM

External

Thermistor

EP

C8

Q3

Q4

+

R11

R12

R14

R16

Rrestart

C13

+

C15

R13

GND

DIM

EN

Rhys

Rth

-

LM3431

SNVS547F – NOVEMBER 2007 – REVISED MARCH 2009

www.ti.com

Figure 14. Typical Application Schematic

OPERATION DESCRIPTION

The LM3431 combines a boost controller and 3 constant current regulator controllers in one device. To simplify

the description, these two blocks will be described separately as Boost Controller and LED Current Regulator. All

descriptions and component numbers refer to the Figure 14 schematic. The LED bottom cathode nodes (VC1 –

VC4) are referred to simply as the cathode.

BOOST CONTROLLER

The LM3431 is a current-mode, PWM boost controller. Although the LM3431 may be operated in either

continuous or discontinuous conduction mode, the following guidelines are designed for continuous conduction

operation. This mode of operation gives lower output ripple and better LED current regulation.

In continuous conduction mode (when the inductor current never reaches zero), the boost regulator operates in

two cycles. In the first cycle of operation, the NFET is turned on and current ramps up and is storing energy in

the inductor. During this cycle, diode D1 is reverse biased and load current is supplied by the output capacitors -

C8 and C9 in Figure 14.

In the second cycle, the NFET is off and the diode is forward biased. Inductor current is transferred to the load

and output capacitor. The ratio of these two cycles determines the output voltage and is expressed as D or D’:

(1)

where D is the duty cycle of the switch.

(2)

Maximum duty cycle is limited to 85% typically.

10

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