LT3472

8

3472f

and larger capacitance, which can cause significant switch-

ing losses at 1.1MHz switching frequency. Some recom-

mended Schottky diodes are listed in Table 4.

Table 4. Recommended Schottky Diodes

Forward

Forward

Diode

Current

Voltage

Capacitance

Part No.

(mA)

Drop (V)

(pF)

Manufacturer

CMDSH-3

100

0.58 @100mA

7 @ 10V

Central Semiconductor

CMDSH2-3

200

0.49 @ 200mA 15 @ 10V

(631) 435-1110

www.centralsemi.com

Setting the Output Voltages

The LT3472 has an accurate feedback resistor of 50k for

each channel. Only one resistor is needed to set the output

voltage for each channel. The output voltage can be set

according to the following formulas:

V

R

k

V

R

k

POS

NEG

=+

⎛

⎝

⎜

⎞

⎠

⎟

=

⎛

⎝

⎜

⎞

⎠

⎟

125

1

1

50

125

2

50

.•

–.

•

In order to maintain accuracy, high precision resistors are

preferred (1% is recommended).

Soft-Start

The LT3472 has independent soft-start control for each

channel. As shown in Figure 1, the SSP and SSN pins have

an internal resistor of 50k pulling up to 1.25V, respec-

tively. By connecting a capacitor from the SSP or SSN pin

to ground, the ramp of each output can be programmed

individually. If SSP or SSN is open or pull higher than

1.25V, the corresponding output will ramp up quickly. The

waveforms with and without soft-start for the Boost

channel are shown in Figure 4.

The waveforms with and without soft-start for the negative

channel are shown in Figure 5.

Start Sequencing

The LT3472 has internal sequencing circuitry that inhibits

the negative channel from operating until feedback voltage

of the step-up channel reaches about 1.1V, ensuring that

APPLICATIO S I FOR ATIO

2V/DIV

5V/DIV

200mA/DIV

100µs/DIV

3472 FO4b

1V/DIV

5V/DIV

100mA/DIV

500µs/DIV

3472 FO5a

2V/DIV

5V/DIV

200mA/DIV

100µs/DIV

3472 FO5b

1V/DIV

5V/DIV

100mA/DIV

1ms/DIV

3472 FO4a