LT1640L/LT1640H
APPLICATIONS INFORMATION
The waveforms are shown in Figure 6b. When the power
pins make contact, they bounce several times. While the
contacts are bouncing, the LT1640 senses an undervoltage
condition and the GATE is immediately pulled low when
the power pins are disconnected.
Once the power pins stop bouncing, the GATE pin starts to
ramp up. When Q1 turns on, the GATE voltage is held
constant by the feedback network of R3 and C2. When the
DRAIN voltage has finished ramping, the GATE pin then
ramps to its final value.
Electronic Circuit Breaker
The LT1640 features an electronic circuit breaker function
that protects against short circuits or excessive supply
currents. By placing a sense resistor between the V
EE
and
SENSE pin, the circuit breaker will be tripped whenever the
voltage across the sense resistor is greater than 50mV for
more than 3µs as shown in Figure 7.
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:
V( t) – V( t
O
)
t
=
–R • C • In
1 –
V
i
– V( t
O
)
Figure 7. Short-Circuit Protection Waveforms
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
8
U
W
U
U
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 over current condition occurs. V
i
is the
voltage across the sense resistor when the short current
or over current 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.
GND
1640 F07
R4
562k
1%
UV = 37V
R5
9.09k
1%
R6
10k
1%
8
V
DD
3
UV
LT1640L /LT1640H
2
OV
V
EE
4 C3
SENSE
5
C1
0.033µF
24V
GATE
6
R2 R3
10Ω 10k C2
5% 5% 3.3nF
100V
1640 F08
PWRGD/
PWRGD
DRAIN
7
1
OV = 71V
+
C
L
100µF
100V
R1
0.02Ω
5%
– 48V
R7
Q1
IRF530
Figure 8. Extending the Short-Circuit Protection Delay
LINER [ LINEAR TECHNOLOGY ]
