LT1432CS8 datasheet(25/28 Pages) LINER

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LT1432

µs/DIV

POSITIVE TO EGATIVE CO VERTER

INPUT VOLTAGE (V)

1.0

1.5

2.0

2.5

3.0

510

LT1432 F23

TJ = 25°C

RLIM = 0.02Ω

VOUT = 0 (SHORT CIRCUIT)

∆VOUT = 1%

IDIODE

1A/DIV

ISWITCH

1A/DIV

Figure 23. Positive-to-Negative Converter

Output Current

tor current is diverted through the catch diode to the

output. Figure 24 shows switch current (1A/DIV) with the

upper waveform, and catch diode current (which is deliv-

ered to the output) in the lower waveform, with a +5V input

and 1A load. Note that switch, inductor, and diode currents

are much higher than output current as required by the fact

that current is delivered to the output during only part of

a switch cycle. An approximate formula for peak switch

current required in an inverting design is:

The first term (2.29A) represents the minimum switch

current required if the inductor were infinitely large. A

finite inductor value requires additional switch current.

The 0.4A represents one-half the peak-to-peak inductor

ripple current. The end result is that peak switch current is

almost three times output load current. This multiplier

drops rapidly at higher input voltages, so worst case is

calculated at lower input voltage.

Figure 25 shows the efficiency of this converter. At higher

input voltages and modest output currents efficiency

hovers around 85%, quite good for a 5V output inverter.

Low input voltage reduces efficiency because of increased

currents in the switch, catch diode, and inductor. High

input voltage and low output current also show lower

efficiency due to quiescent currents in the ICs. Note that

the efficiency is actually significantly improved in this

regard over a more conventional design because the

VF = Forward voltage of catch diode

RSW = Switch on-resistance

L = Inductor value

f = Switching frequency

If VIN is 4.7V (minimum),

VF = 0.4V, RSW = 0.25Ω,

L = 50

µH, f = 60kHz, and IOUT = 1A;

Figure 24. Positive-to-Negative Converter

Switch and Diode Current

V– I

2L f V

SW PEAK

OUT

() =+

()













()

()( ) +

()

I1 1

0.4

4.7 – 1 0.25

4.7

4.75 5

2 50E

60E

4.75 5

2.29

0.4

2.69A

SW PEAK

–6

() =+

()

















()

















()

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