LT3472

6

3472f

APPLICATIO S I FOR ATIO

Operation

The LT3472 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 X1 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 X1 is reset turning off the power switch

Q1. 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 1.25V. In this manner, the error amplifier sets

the correct peak current level to keep the output in regu-

lation. If the error amplifier’s output increases, more

current is delivered to the output; if it decreases, less

current is delivered. The second channel is an inverting

converter. The basic operation is the same as the positive

channel. The SR latch X2 is also set at the start of each

oscillator cycle. The power switch Q2 is turned on at the

same time as Q1. The turn off of Q2 is determined by its

own feedback loop, which consists of error amplifier A3

and PWM comparator A4. The reference voltage of this

negative channel is ground.

Switching waveforms with typical load conditions are

shown in Figure 2.

Inductor Selection

A 22µH inductor is recommended for LT3472 step-up

channel. The inverter channel can use a 22µH or 47µH

inductor. 47µH inductors will provide slightly more cur-

rent. Small size and high efficiency are the major concerns

for most LT3472 applications. Inductors with low core

losses and small DCR (copper wire resistance) at 1.1MHz

are good choices for LT3472 applications. Some induc-

tors in this category with small size are listed in

Table 1. The efficiency comparison of different inductors

is shown in Figure 3.

Figure 3. Efficiency Comparison of Different Inductors

0

85

80

75

70

65

60

5

10

15

20

3473 F02a

25

30

LQH2MCN220

LQH32CN220

TOKO 1067FB-220M

3472 F02a

INVERTER LOAD = 20mA

0

85

80

75

70

65

60

5

10

15

20

3473 F02a

25

30

LQH2MCN220

LQH32CN220

TOKO 1067FB-220M

3472 F02b

BOOST LOAD = 20mA

20V/DIV

100mA/DIV

20V/DIV

100mA/DIV

500ns/DIV

3472 FO4

Figure 2. Switching Waveforms