LT3476EUHF(RevA) Просмотр технического описания (PDF) - Linear Technology

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производитель

LT3476EUHF
(Rev.:RevA)

High Current Quad Output LED Driver

Linear Technology 

LT3476

BLOCK DIAGRA

EXTERNAL COMPONENTS

RSNS (EXT)

BUCK MODE

PVIN

0.1Ω

33V

CAP

LED

3, 4, 9, 10

2, 5, 8, 11

CBYP

2.2μF

LED ARRAY

CFILT

0.1μF

10μH

SW

22-29

RSET

2kΩ

VADJ

14, 15,

36, 37

1.25V +

+ A4

–

PWM

16, 17,

34, 35

IDLE MODE

VC

1, 12, 13, 38

+

A1

–

ERROR

AMPLIFIER

THERMAL

Q3 LIMIT

145°C

Q2

RSET1

20kΩ

35V

OVERVOLTAGE

DETECT

25k

PWM

–

A2

+

PWM

COMPARATOR

∑

VIN

VIN

33

3V

REF

7

–

+

1.05V

ISRC

300μA

V1

Q4

RAMP

GENERATOR

200kHz

to 2MHz

OSCILLATOR

RT

6

NC

19, 20, 21

30, 31, 32

DRIVER

RQ

S

A3

Q1

MAIN

SWITCH

RSW

0.02Ω

CURRENT SENSE

AMPLIFIER

LT3476 CHANNEL

SHUTDOWN

SHDN

18

3476 BD

U

OPERATIO

The LT3476 is a constant-frequency, current mode regula-

tor with an internal power switch. Operation can be best

understood by referring to the Block Diagram. At the

start of each oscillator cycle, the SR latch is set, which

turns on the Q1 power switch. A voltage proportional to

the switch current is added to a stabilizing ramp and the

resulting sum is fed into the positive terminal of the PWM

comparator, A2. When this voltage exceeds the level at

the negative input of A2, the SR latch is reset, turning off

the power switch. The level at the negative input of A2 is

set by the error amplifier A1, and is simply an amplified

version of the difference between the voltage across the

internal resistor RSET and the voltage across the external

current sense resistor RSNS. In this manner, the error

amplifier sets the correct peak switch current level to

regulate the current through RSNS. If the error amplifier’s

output increases, more current is delivered to the output;

if it decreases, less current is delivered.

The current regulated in RSNS can be adjusted by chang-

ing the voltage across RSET using the VADJ input pin. The

amplifier A4 regulates current in Q3 to produce a voltage

across RSET equal to VADJ. This current flowing through

transistor Q3 also produces a voltage across RSET one-

tenth the magnitude of the VADJ input and level shifted

to the CAP input. The voltage across RSET is limited to

125mV (typ) by the separate 1.25V input on A4.

The average current regulated in RSNS can also be adjusted

for dimming using the PWM pin. When the PWM pin is

low, switching is disabled and the error amplifier is turned

off so that it does not drive the VC pin. Also, all internal

loads on the VC pin are disabled so that the charge state

of the VC pin will be saved on the external compensation

capacitor. This feature reduces transient recovery time

because when the PWM input again transitions high, the

demand current for the switch returns to the value just

before PWM last transitioned low.

3476fa

6

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