LT3460/LT3460-1

5

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BLOCK DIAGRAM

Figure 1. Block Diagram, LT3460

–

+

–

+

–

+

R1 (EXTERNAL)

R2 (EXTERNAL)

FB

SHUTDOWN

SHDN

RAMP

GENERATOR

1.255V

REFERENCE

1.3MHz

OSCILLATOR

R

S

Q

A1

A2

COMPARATOR

DRIVER

SW

Q1

0.1Ω

GND

FB

3460 BD

OPERATION

The LT3460/LT3460-1 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 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.

Feedback Loop Compensation

The LT3460/LT3460-1 has an internal feedback compensa-

because the small signal characteristics of a boost converter

change with operation conditions, the internal compensa-

tion network cannot satisfy all applications. A properly

most applications.

The LT3460/LT3460-1 uses peak current mode control.

The current feedback makes the inductor very similar

to a current source in the medium frequency range. The

power stage transfer function in the medium frequency

range can be approximated as:

K1

where C2 is the output capacitance, and K1 is a constant

based on the operating point of the converter. In continuous

current mode, K1 increases as the duty cycle decreases.

proximated as follows in medium frequency range:

The zero

1

is about 70kHz.