LT1640AH LINER [Linear Technology], LT1640AH Datasheet - Page 9

• Manufacturer Part Number: LT1640AH
• Description: Negative Voltage Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
and SENSE pin, the circuit breaker will be tripped when-
ever the voltage across the sense resistor is greater than
50mV for more than 3 s 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
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 3 s 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 20 A, there will be a voltage drop
Figure 7. Short-Circuit Protection Waveforms
EE
and the external N-channel turns off. The
U
U
W
U
1640A F07
EE
UV = 37V
OV = 71V
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 100 for R7
will cause a 2mV error. The following equation can be used
to estimate the delay time at the SENSE pin:
Where V(t) is the circuit breaker trip voltage, typically
50mV. V(t
before the short or overcurrent condition occurs. V
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 0.02
sense resistor is used. An extended delay circuit needs to
be designed for a 50 s delay time after the load jumps to
5A. In this case:
If we choose R = 100 , we will get C = 1 F.
* DIODES INC. SMAT70A
– 48V
GND
V(t) = 50mV
V(t
V
t
*
i
Figure 8. Extending the Short-Circuit Protection Delay
= 5A • 0.02 = 100mV
O
GND
– • •
) = 20mV
9.09k
R C In
562k
10k
1%
1%
1%
R4
R5
R6
(SHORT PIN)
O
) is the voltage drop across the sense resistor
3
2
UV
OV
V
3
EE
4
1
0.02
1
5%
C3
R1
LT1640AL/LT1640AH
–
V t
SENSE
2
V
( ) – ( )
i
4
5
R7
– ( )
V t
LT1640AL
V t
C1
150nF
25V
GATE
V
O
DD
O
8
6
IRF530
Q1
R2
10
5%
18k
5%
R3
3.3nF
100V
C2
PWRGD
DRAIN
7
1
+
i
is the
1640A F08
9
C
100 F
100V
L