LT1618

5

sn1618 1618fas

1.4MHz

OSCILLATOR

DRIVER

–

+

–

+

+

+

–

–

+

×5

Σ

Q

Q1

S

R

–

+

A1

×25

5

10

3

2

8

7

9

1.263V

4

1

SHDN

GND

SW

ISN

ISP

FB

A2

A3

0.02Ω

C1

C2

R1

R2

L1

D1

Figure 1. LT1618 Block Diagram

BLOCK DIAGRA

OPERATIO

The LT1618 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 power switch Q1. The

signal at the noninverting input of PWM comparator A3 is

a scaled-down version of the switch current (summed

together with a portion of the oscillator ramp). When this

signal reaches the level set by the output of error amplifier

A2, comparator A3 resets the latch and turns off the power

switch. In this manner, A2 sets the correct peak current

level to keep the output in regulation. If the error amplifier’s

output increases, more current is delivered to the output;

if it decreases, less current is delivered. A2 has two

inverting inputs, one from the voltage feedback loop, and

one from the current feedback loop. Whichever inverting

input is higher takes precedence, forcing the converter

into either a constant-current or a constant-voltage mode.

The LT1618 is designed to transition cleanly between the

two modes of operation. Current sense amplifier A1 senses

the voltage between the ISP and ISN pins and provides a

25× level-shifted version to error amplifier A2. When the

voltage between ISP and ISN reaches 50mV, the output of

A1 provides 1.263V to one of the noninverting inputs of A2

and the converter is in constant-current mode. If the

current sense voltage exceeds 50mV, the output of A1 will

increase causing the output of A2 to decrease, thus

reducing the amount of current delivered to the output. In

this manner the current sense voltage is regulated to

50mV. Similarly, if the FB pin increases above 1.263V, the

output of A2 will decrease to reduce the peak current level

and regulate the output (constant-voltage mode).