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LT1432 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
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U
U
POSITIVE TO EGATIVE CO VERTER
LT1432
3.0
TJ = 25°C
RLIM = 0.02
2.5
2.0
VOUT = 0 (SHORT CIRCUIT)
1.5
VOUT = 1%
1.0
ISWITCH
1A/DIV
0
2
1
0
5
10
15
20
25
INPUT VOLTAGE (V)
LT1432 F23
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:
( ) ( ) ( ) ISW PEAK
=
IOUT
1
+

VIN
VOUT +
IOUT RSW
VF
VIN
+ VOUT
VIN

+
( ) VIN VOUT
2(L)(f)(VIN + VOUT
)
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 = 0.25,
L = 50µH, f = 60kHz, and IOUT = 1A;
IDIODE
1A/DIV
0
5µs/DIV
Figure 24. Positive-to-Negative Converter
Switch and Diode Current
( ) ( ) ( ISW PEAK)
= 11+

4.7 – 1
5 + 0.4
4.7 + 5
0.25
4.7

+
4.75(5)
( ) 2
50E–6
60E3
4.75 + 5
= 2.29 + 0.4 = 2.69A
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
25
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