LT1432 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
LT1432
APPLICATI S I FOR ATIO
because these traces do not have errors caused by plating
operations. The suggested trace width for 1/2oz foil is
0.03" for each 1A of current limit to keep trace temperature
rise reasonable. 3A current limit would require the width
to be 0.09". 1oz foil can reduce trace width to 0.02" per
amp. Inductance in the trace is not critical so the trace can
be wound serpentine or any other shape that fits available
space. Kelvin connections should be used as shown in
Figure 14 to avoid errors due to termination resistance.
The length of the sense resistor trace can be calculated
from:
( ) Length = W RSENSE Inches
RCU
W = width of copper trace (≈0.03" per amp for 1/2oz
copper foil)
RCU = resistivity of PCB trace, expressed as Ω per square.
It is found by calculating the resistance of a section of trace
with equal length and width. For typical 1/2oz material,
RCU is approximately 1mΩ per square. In the example
shown above, with width = 2A times 0.03" = 0.06";
( ) 0.06 0.02
Length =
= 1.2 Inches
0.001
Current limiting maintains true switching action, but power
dissipation in the IC switch and catch diode will shift
depending on output voltage. At output voltages near the
correct regulated value, power will be distributed between
switch and the diode according to the usual calculations.
Under short circuit conditions, switch duty cycle will drop
to a very low value, and power will concentrate in the
diode, which will be running at near 100% duty cycle. If
continuous shorts must be tolerated, the catch diode must
be sized to handle the full current limit value, or foldback
current can be used.
Foldback Current Limiting
Foldback current limiting makes the short circuit current
limit somewhat lower than the full load current limit to
reduce component stress under short circuit conditions.
This is shown in Figure 15 with the addition of R3 and R4.
The voltage drop across R3 adds to the 60mV current limit
VIN
6V – 25V
+
C1
200µF
35V
VSW
VIN
LT1271
FB
VC
GND
C6
0.02µF
C4
0.1µF
R1
680Ω
C5
0.03µF
C3 +
4.7µF
TANT
D2
1N4148
L1
50µH
R2
0.025Ω
D1
MBR330p
<0.3V = NORMAL MODE
>2.5V = SHUTDOWN
OPEN = BURST MODE
220pF
VC DIODE
VIN
LT1432
MODE
GND
V+
VLIM
VOUT
R3
100Ω
R4
12.5k
Figure 15. Adding Foldback Current Limiting
10µH
3A
+
100µF
16V
OPTIONAL
OUTPUT
FILTER
+
× VOUT
5V
C2
3A
470µF
16V
LT1432 F15
14
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