9
LT1640AL/LT1640AH
V
EE
V
DD
LT1640AL
PWRGD
SENSE
C1
150nF
25V
C
L
100µF
100V
Q1
IRF530
R2
10Ω
5%
R3
18k
5%
C2
3.3nF
100V
R4
562k
1%
R5
9.09k
1%
R6
10k
1%
R1
0.02Ω
5%
4
3
2
OV = 71V
GND
–48V
UV = 37V
OV
UV
5
C3
6
8
1
GATE DRAIN
1640A F08
+
7
GND
(SHORT PIN)
*
* DIODES INC. SMAT70A
R7
43
21
Figure 8. Extending the Short-Circuit Protection DelayFigure 7. Short-Circuit Protection Waveforms
1640A F07
Electronic Circuit Breaker
The LT1640A features an electronic circuit breaker func-
tion that protects against short circuits or excessive sup-
ply currents. By placing a sense resistor between the V
EE
and SENSE pin, the circuit breaker will be tripped when-
ever the voltage across the sense resistor is greater than
50mV for more than 3µs as shown in Figure 7.
Note that the circuit breaker threshold should be set
sufficiently high to account for the sum of the load current
and the inrush current. If the load current can be controlled
by the PWRGD/PWRGD pin (as in Figure 6a), the threshold
can be set lower, since it will never need to accommodate
inrush current and load current simultaneously.
When the circuit breaker trips, the GATE pin is immediately
pulled to V
EE
and the external N-channel turns off. The
GATE pin will remain low until the circuit breaker is reset
by pulling UV low, then high or cycling power to the part.
If more than 3µs deglitching time is needed to reject
current noise, an external resistor and capacitor can be
added to the sense circuit as shown in Figure 8. R7 and C3
act as a lowpass filter that will slow down the SENSE pin
voltage from rising too fast. Since the SENSE pin will
source current, typically 20µA, there will be a voltage drop
on R7. This voltage will be counted into the circuit breaker
trip voltage just as the voltage across the sense resistor.
A small resistor is recommended for R7. A 100Ω for R7
will cause a 2mV error. The following equation can be used
to estimate the delay time at the SENSE pin:
tRCIn
Vt Vt
VVt
O
iO
=
–•• –
()– ( )
–()
1
Where V(t) is the circuit breaker trip voltage, typically
50mV. V(t
O
) is the voltage drop across the sense resistor
before the short or overcurrent condition occurs. V
i
is the
voltage across the sense resistor when the short current
or overcurrent is applied on it.
Example: A system has a 1A current load and a 0.02Ω
sense resistor is used. An extended delay circuit needs to
be designed for a 50µs delay time after the load jumps to
5A. In this case:
V(t) = 50mV
V(t
O
) = 20mV
V
i
= 5A • 0.02Ω = 100mV
If we choose R = 100Ω, we will get C = 1µF.
APPLICATIO S I FOR ATIO
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