LT1432 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
LT1432
U
U
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
of input voltage in Figure 23. A 0.02Ω 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-
INPUT
4.5V – 25V
C1 +
330µF
35V
VSW
VIN
FB
GND
C6
0.02µF
LT1271
VC
C5
0.03µF
D2
1N4148
D1
MBR330p
R1
680Ω C4
0.047µF
C3 +
22µF
16V
L1
50µH
R4
0.02Ω
DIODE VC
VIN
LT1432
MODE
V+
VLIM
VOUT
GND
R2
100Ω
R3
100k
+ C2
1000µF
16V
× × –5V
OUTPUT
Figure 22. Positive-to-Negative Converter
10µH
3A
100µF
16V
OPTIONAL
OUTPUT
FILTER
LT1432 F22
24
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