LT1432CS8 datasheet(24/28 Pages) LINER

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LT1432

POSITIVE TO EGATIVE CO VERTER

The circuit in Figure 22 will convert a variable positive input

voltage to a regulated –5V output. By selecting different

members of the LT1070 family, this basic design can

provide up to 6A output current at high input voltages, and

up to 3A with a five volt input supply. As shown using an

LT1271, maximum load current has been reduced to 1A by

utilizing the current limit circuit in the LT1432. Unlike a

positive buck converter, it is not possible to sense output

current directly. Instead, switch/inductor current is sensed.

This would normally result in a DC output current limit

value that changes considerably with input voltage, but the

addition of R2 and R3 alters peak current limit as a function

of input voltage to correct for this effect. Maximum load

current and short circuit current are shown as a function

Figure 22. Positive-to-Negative Converter

of input voltage in Figure 23. A 0.02

Ω sense resistor was

used, so other values of current limit can be scaled from

this value.

This circuit uses the same basic connections between the

LT1432 and the LT1271 as the buck converter. The differ-

ence is in the way power flows in the catch diode, inductor,

and switch. In a buck converter, current flows simulta-

neously in the switch, inductor, and output. This makes

maximum output current approximately equal to maxi-

mum switch current. In inverting designs, current deliv-

ered to the output is zero during switch on-time. The

switch allows current to flow directly from the input supply

through the inductor to ground. At switch turn-off, induc-

VSW

GND

VIN

LT1271

330

µF

35V

µF

16V

680

Ω

0.02

µF

1N4148

1000

µF

16V

µH

INPUT

4.5V – 25V

MBR330p

0.047

µF

100k

100

Ω

0.03

µF

× ×

OPTIONAL

OUTPUT

FILTER

100

µF

16V

µH

–5V

OUTPUT

LT1432 F22

0.02

Ω

GND

VIN

MODE

DIODE

VOUT

VLIM

V+

LT1432

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