Datasheet

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     
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SLUS384B – NOVEMBER 1999 – REVISED JUNE 2000
5
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APPLICATION INFORMATION
98
54 1312
10
6
7
3
2
1
16
15
14
VIN
VIN
VIN
VOUT
VOUT
VOUT
SHTDWN
FAULT
CT
HEAT SINK GND
PINS
GND
IFAULT IMAX
R
IMAX
R
IFAULT
C
T
C
SD
R
SD
V
IN
S6
R
L
C
OUT
V
OUT
C
IN
D1
R1
V
IN
UDG–99152
Figure 1. Typical Application
protecting the UCC3918 from voltage transients
The parasitic inductance associated with the power distribution can cause a voltage spike at V
IN
if the load
current is suddenly interrupted by the UCC3918. It is important to limit the peak of this spike to less than 6 V
to prevent damage to the UCC3918. This voltage spike can be minimized by:
• Reducing the power distribution inductance (e.g., twist the positive “+” and negative “–” leads of the
power supply feeding V
IN
, locate the power supply close to the UCC3918 or use a PCB ground plane).
• Decoupling V
IN
with a capacitor, C
IN
(refer to Figure 1), located close to the VIN pin. This capacitor is
typically less than 1 µF to limit the inrush current.
• Clamping the voltage at V
IN
below 6 V with a Zener diode, D1 (refer to Figure 1), located close to the VIN
pin.
estimating maximum load capacitance
For circuit breaker applications, the rate at which the total output capacitance can be charged depends on the
maximum output current available and the nature of the load. For a constant-current current-limited circuit
breaker, the output comes up if the load requires less than the maximum available short-circuit current.
To ensure recovery of a duty-cycle of the current-limited circuit breaker from a short-circuited load condition,
there is a maximum total output capacitance that can be charged for a given unit ON time (fault time). The design
value of ON or fault time can be adjusted by changing the timing capacitor C
T
.