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LT3460

3460f

The LT3460 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:

()

•

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

based on the operating point of the converter. In continu-

ous current mode, K1 increases as the duty cycle de-

creases.

The internal compensation network RC, CC can be approxi-

mated as follows in medium frequency range:

()

•

••

•

The zero

2•

•

OPERATIO

Feedback Loop Compensation

The LT3460 has an internal feedback compensation net-

work as shown in Figure 1 (RC and CC). However, because

the small signal characteristics of a boost converter change

with operation conditions, the internal compensation net-

work cannot satisfy all applications. A properly designed

external feed forward capacitor from VOUT to FB (CF in

Figure 2) will correct the loop compensation for most

applications.

In order to improve the phase margin, a feed-forward

capacitor CF in Figure 2 can be used.

Without the feed-forward capacitor, the transfer function

from VOUT to FB is:

OUT

With the feed-forward capacitor CF, the transfer function

becomes:

OUT

•

••

•

The feed-forward capacitor CF generates a zero and a pole.

The zero always appears before the pole. The frequency

distance between the zero and the pole is determined only

by the ratio between VOUT and FB. To give maximum phase

FREQUENCY (kHz)

PHASE

GAIN

–10

–20

–30

–40

–45

–90

–135

–180

–225

–270

–315

–360

100

1000

3460 F03

is about 70kHz.

The feedback loop gain T(s) = K3 • GP(s) • GC(s). If it

crosses over 0dB far before fZ, the phase margin will be

small. Figure 3 is the Bode plot of the feedback loop gain

measured from the converter shown in Figure 2 without

the feedforward capacitor CF. The result agrees with

the previous discussion: Phase margin of about 20

° is

insufficient.

OFF ON

VIN

µH

130k

15k

µF

22pF

4.7

µF

VOUT

12V

70mA

VIN

SHDN

GND

LT3460

C1: TAIYO YUDEN X5R JMK212BJ475KG

C2: TAIYO YUDEN X5R EMK316BJ105

D1: CENTRAL SEMICONDUCTOR CMDSH2-3

L1: MURATA LQH32CN-220 OR EQUIVALENT

3460 F02

Figure 2. 5V to 12V Step-Up Converter

Figure 3

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