LT1432CS8 datasheet(26/28 Pages) LINER

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LT1432

POSITIVE TO EGATIVE CO VERTER

LT1271 operates from a constant 5V supply voltage rather

than the high input voltage.

Output voltage ripple in an inverter can be much higher

than a buck converter because current is delivered to the

output capacitor in high amplitude square waves rather

than a DC level with superimposed tri-wave. C2 is there-

fore somewhat larger than in a buck design. Also C2 must

be rated to handle the large RMS current pulses fed into it.

This RMS current is approximately equal to:

OUT













For 1A output current, with 5V input, this computes to

1ARMS in the output capacitor. A small additional output

filter would reduce output ripple voltage, but it does not

change the current rating requirement for the main output

capacitor. The reader is referred to a switching regulator

CAD program (SwitcherCAD) supplied by LTC for further

insight into converters. It is suggested that the reader fool

the program by asking for a negative input, positive output

design. It will then select the LT1070 family of ICs which

normally are not used in positive to negative converters.

Efficiency calculations will be somewhat in error at higher

input voltages because the program assumes full input

voltage across the IC. Later versions of SwitcherCAD will

have a special section for this particular design.

Figure 25. Positive-to-Negative Converter Efficiency

OUTPUT CURRENT (A)

100

0.8

LT1432 F25

0.2

0.4

0.6

1.0

VIN = 5V

VIN = 10V

VIN = 20V

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