Datasheet

LT4256-1/LT4256-2
11
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APPLICATIO S I FOR ATIO
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When the TIMER pin reaches 4.65V (typ), the internal fault
latch is set causing GATE to be pulled low and TIMER to be
discharged to GND by the 3µA current source. The part is
not allowed to turn on again until the voltage on TIMER
falls below 0.65V (typ).
TIMER must never be pulled high by a low impedance
because whenever TIMER rises above the upper threshold
(typically 4.65V) the pin characteristics change from a
high impedance current source to a low impedance.
Whenever GATE is commanded off by any fault condition,
it is discharged rapidly, turning off the external MOSFET.
The waveform in Figure 9 shows how the output latches off
following a current fault (LT4256-1). The drop across the
sense resistor is held at 55mV as the timer ramps up. Once
TIMER reaches its shutdown threshold (4.65V typically),
the circuit latches off.
The LT4256-1 latches off after a current limit fault. After
the LT4256-1 latches off, the part may be commanded to
start back up. This is accomplished by cycling UV to
ground and then back high (this command can only be
accepted after TIMER discharges back below the 0.65V
typical threshold, to prevent overheating transistor Q1).
Automatic Restart
The LT4256-2 will automatically restart after an overcurrent
fault. These waveforms are shown in Figure 10.
The LT4256-2 functionality is as follows: When an
overcurrent condition occurs, the GATE pin is servoed to
maintain a constant voltage across the sense resistor, and
the capacitor C2 at the TIMER pin will begin to charge.
When the voltage at the TIMER pin reaches 4.65V (typ),
the GATE pin is pulled low. When the voltage at the TIMER
pin ramps back down to 0.65V (typ), the LT4256-2 turns
on again. If the short-circuit condition at the output still
exists, the cycle will repeat itself indefinitely. The duty
cycle under short-circuit conditions is 3% which prevents
Q1 from overheating.
Figure 9. LT4256-1 Current Limit Waveforms
Figure 10. LT4256-2 Current Limit Waveforms
10ms/DIV
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I
OUT
500mA/DIV
V
OUT
50V/DIV
TIMER
5V/DIV
GATE
50V/DIV
I
OUT
500mA/DIV
V
OUT
50V/DIV
TIMER
5V/DIV
GATE
50V/DIV
10ms/DIV
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