LT1641IS8 Ver la hoja de datos (PDF) - Linear Technology
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LT1641IS8 Datasheet PDF : 12 Pages
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LT1641
APPLICATIO S I FOR ATIO
When an overcurrent condition occurs, the GATE pin is
driven to maintain a constant voltage across the sense
resistor. The capacitor C2 at the TIMER pin will begin to
charge. When the voltage at the TIMER pin reaches
1.233V, the GATE pin is immediately pulled to GND and
transistor Q1 turns off. Capacitor C1 momentarily pulses
the ON pin low and clears the internal fault latch. When the
voltage at the TIMER pin ramps back down to 0.5V, the
LT1641 turns on again. If the short-circuit condition at the
output still exists, the cycle will repeat itself indefinitely
with a 3.75% on-time duty cycle which prevents Q1 from
overheating. The waveforms are shown in Figure 11.
After the TIMER pin is pulled higher than 1.233V, the fault
latch is set and the GATE pin is pulled to GND immediately,
turning off transistor Q1. The waveforms are shown in
Figure 13. Operation is restored either by interrupting
power or by pulsing ON low.
VIN
24V
R1
49.9k
1%
R2
3.4k
1%
GND
RS
Q1
0.025Ω IRF530
D1
30V
1N5256B
8
VCC
1
ON
D1
R5
CMPZ
10Ω
5248B
5%
C1
R6, 10nF
1k, 5%
76
SENSE GATE
2
FB
LT1641
TIMER
GND
5
4
C2
0.68µF
3
PWRGD
+
VOUT
R3
CL
59k
1%
R7
24k
5%
R4
3.57k
1%
PWRGD
1641 F12
Figure 12. Overvoltage Detection
Figure 11. Automatic Restart Waveforms
Undervoltage and Overvoltage Detection
The ON pin can be used to detect an undervoltage condi-
tion at the power supply input. The ON pin is internally
connected to an analog comparator with 80mV of hyster-
esis. If the ON pin falls below its threshold voltage (1.233V),
the GATE pin is pulled low and is held low until ON is high
again.
Figure 12 shows an overvoltage detection circuit. When
the input voltage exceeds the Zener diode’s breakdown
voltage, D1 turns on and starts to pull the TIMER pin high.
Figure 13. Overvoltage Waveforms
9
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