LT3462/LT3462A

3462af

4

OPERATIO

BLOCK DIAGRA

Figure 1. Block Diagram

–

+

–

+

–

+

–

+

1

3

2

∑

D

R1 (EXTERNAL)

R2 (EXTERNAL)

FB

RAMP

GENERATOR

SHUTDOWN

BIAS CURRENT

CANCELLATION

1.2MHz*

OSCILLATOR

R

S

Q

A1

E AMP

A2

COMP

DRIVER

SW

Q1

0.1

Ω

GND

6

1.265V

REFERENCE

FB

4

SDREF

SDREF

SDREF

3462 F02

5

*LT3462A IS 2.7MHz

Q2

OFF

→ 3µA

ON

→ 180µA

ISRC

SHUTDOWN

DO

LG

The LT3462 uses a constant frequency, current mode

control scheme to provide excellent line and load regula-

tion. Operation can be best understood by referring to the

Block Diagram in Figure 1. At the start of each oscillator

cycle, the SR latch is set, turning on the power switch Q1.

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. When this

voltage exceeds the voltage at the output of the EAMP, the

SR latch is reset, turning off the power switch. The level

at the output of the EAMP is simply an amplified version

of the difference between the feedback voltage and GND.

In this manner, the error amplifier sets the correct peak

current level to keep the output in regulation. If the error

amplifier’s output increases, more current is taken from

the output; if it decreases, less current is taken. One

function not shown in Figure 1 is the current limit. The

switch current is constantly monitored and not allowed to

exceed the nominal value of 400mA. If the switch current

reaches 400mA, the SR latch is reset regardless of the

output state of the PWM comparator. This current limit cell

protects the power switch as well as various external

components connected to the LT3462.

SDREF is a dual function input pin. When driven low it

shuts the part down, reducing quiescent supply current to

less than 10

µA. When not driven low, the SDREF pin has an

internal pull-up current that turns the regulator on. Once

the part is enabled, the SDREF pin sources up to

180

µA nominally at a fixed voltage of 1.265V through

external resistor R2 to FB. If there is no fault condition

1.265V • (–R1/R2). An optional soft-start circuit uses the

fixed SDREF pull-up current and a capacitor from SDREF

current cancellation circuit supplies up to 150

µA biasing

This function eliminates R2 loading of SDREF during

shutdown. As a result, supply current in shutdown may

exceed 10

µA by the amount of current flowing in R1.