LT3461/LT3461A

4

3461af

OPERATIO

Figure 2. Suggested Layout

R2

R1

GND

C3

L1

C1

SHUTDOWN

C2

3461a F03

The LT3461/LT3461A uses a constant frequency, current

mode control scheme to provide excellent line and load

regulation. 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, 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 compara-

tor 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 in-

creases, more current is delivered to the output; if it

decreases, less current is delivered.

Layout Hints

The high speed operation of the LT3461/LT3461A de-

mands careful attention to board layout. You will not get

advertised performance with careless layout. Figure 2

shows the recommended component placement.

BLOCK DIAGRA

–

+

–

+

–

+

6

1

3

2

∑

R1 (EXTERNAL)

R2 (EXTERNAL)

FB

SHUTDOWN

SHDN

RAMP

GENERATOR

1.255V

REFERENCE

3MHz*

OSCILLATOR

R

S

Q

4

A1

A2

COMPARATOR

DRIVER

SW

Q1

0.1Ω

GND

FB

3461a F02

5

*LT3461 IS 1.3MHz

Figure 1. Block Diagram

PI FU CTIO S

SW (Pin 1): Switch Pin. Connect inductor here. Minimize

trace at this pin to reduce EMI.

GND (Pin 2): Ground Pin. Tie directly to local ground

plane.

FB (Pin 3): Feedback Pin. Reference voltage is 1.255V.

Connect resistor divider tap here. Minimize trace area at

SHDN (Pin 4): Shutdown Pin. Tie to 1.5V or higher to

enable device; 0.4V or less to disable device. Also func-

tions as soft-start. Use RC filter (47k, 47nF typ) as shown

in Figure 1.

capacitor close to pin and close to GND plane.