4

LT1930/LT1930A

BLOCK DIAGRA

Figure 2. Block Diagram

OPERATIO

The LT1930 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 2. At the start of each oscillator

cycle, the SR latch is set, which turns 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 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

feedback voltage and the reference voltage of 1.255V. 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 delivered to

the output; if it decreases, less current is delivered. The

LT1930 has a current limit circuit not shown in Figure 2.

The switch current is constantly monitored and not al-

lowed to exceed the maximum switch current (typically

1.2A). If the switch current reaches this value, the SR latch

is reset regardless of the state of comparator A2. This

current limit helps protect the power switch as well as the

external components connected to the LT1930.

The block diagram for the LT1930A (not shown) is iden-

tical except that the oscillator frequency is 2.2MHz.

–

+

RQ

S

0.01

Ω

SW

DRIVER

COMPARATOR

2

SHDN

4

1

FB

3

–

+

Σ

RAMP

GENERATOR

1.255V

REFERENCE

1.2MHz

OSCILLATOR*

GND

1930/A BD

Q1

A2

A1

R1 (EXTERNAL)

R2 (EXTERNAL)

FB

SHUTDOWN

*2.2MHz FOR LT1930A