Datasheet

LT4356-1/LT4356-2
12
4356fb
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Fault Timer Current
The timer current starts at around 2µA with 0.5V or less
of V
DS
, increasing linearly to 50µA with 75V of V
DS
dur-
ing an overvoltage fault (Figure 1). During an overcurrent
fault, it starts at 4µA with 0.5V or less of V
DS
but increases
to 260µA with 80V across the MOSFET (Figure 2). This
arrangement allows the pass transistor to turn off faster
during an overcurrent event, since more power is dissipated
during this condition. Refer to the Typical Performance
Characteristics section for the timer current at different
V
DS
in both overvoltage and overcurrent events.
When the voltage at the TMR pin, V
TMR
, reaches the 1.25V
threshold, the F LT pin pulls low to indicate the detection
of a fault condition and provide warning to the load of
the impending power loss. In the case of an overvoltage
fault, the timer current then switches to a fixed 5µA. The
interval between FLT asserting low and the MOSFET turn
-
ing off is given by:
t
WARNING
=
C
TMR
• 100mV
5µA
ApplicAtions inForMAtion
Figure 1. Overvoltage Fault Timer Current
Figure 2. Overcurrent Fault Timer Current
t
FLT
= 15ms/µF
TOTAL FAULT TIMER = t
FLT
+ t
WARNING
t
WARNING
= 20ms/µF
t
FLT
= 93.75ms/µF t
WARNING
= 20ms/µF
V
TMR(V)
I
TMR
= 5µA I
TMR
= 5µA
V
DS
= 75V
(I
TMR
= 50µA)
V
DS
= 10V
(I
TMR
= 8µA)
1.35
1.25
TIME
4356 F01
0.50
t
FLT
= 2.88ms/µF
TOTAL FAULT TIMER = t
FLT
+ t
WARNING
t
FLT
= 21.43ms/µF
t
WARNING
= 2.86ms/µF
t
WARNING
= 0.38ms/µF
V
TMR(V)
V
DS
= 10V
(I
TMR
= 35µA)
1.35
1.25
0.50
TIME
4356 F02
V
DS
= 80V
(I
TMR
= 260µA)
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