LT4356 Linear Technology Corporation, LT4356 Datasheet - Page 10

• Manufacturer Part Number: LT4356
• Description: Overvoltage Protection Regulator And Inrush Limiter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT4356-1
The interval between FLT asserting low and the MOSFET
turning off is given by:
This fi xed early warning period allows the systems to per-
form necessary backup or house keeping functions before
the power supply is cut off. After V
threshold, the pass transistor turns off immediately. Note
that during an overcurrent event, the timer current is not
reduced to 5μA after V
since it would lengthen the overall fault timer period and
cause more stress on the power MOSFET.
As soon as the fault condition has disappeared, a 2μA
current starts to discharge the timer capacitor to ground.
When V
pump starts to pull the GATE pin high, turning on the
10
1.35
1.25
0.50
1.35
1.25
0.50
V
t
V
TMR(V)
WARNING
TMR(V)
TOTAL FAULT TIMER = t
TOTAL FAULT TIMER = t
= 2.88ms/μF
= 15ms/μF
TMR
t
FLT
t
FLT
Figure 1. Overvoltage Fault Timer Current
Figure 2. Overcurrent Fault Timer Current
(I
reaches the 0.5V threshold, the internal charge
TMR
V
DS
=
(I
= 260μA)
= 80V
TMR
V
t
= 0.38ms/μF
DS
= 20ms/μF
C
t
WARNING
t
WARNING
FLT
t
= 50μA)
FLT
= 75V
TMR
FLT
= 21.43ms/μF
FLT
= 93.75ms/μF
+ t
I
+ t
TMR
5µA
WARNING
WARNING
TMR
• 100mV
= 5μA
has reached 1.25V threshold,
(I
TMR
V
(I
DS
V
TMR
DS
= 35μA)
= 10V
= 10V
= 8μA)
TMR
= 2.86ms/μF
t
WARNING
crosses the 1.35V
= 20ms/μF
t
WARNING
I
TMR
= 5μA
43561 F02
43561 F01
TIME
TIME
MOSFET. The TMR pin is then actively regulated to 0.5V
until the next fault condition appears. The total cool down
timer period is given by:
MOSFET Selection
The LT4356-1 drives an N-channel MOSFET to conduct the
load current. The important features of the MOSFET are
on-resistance R
V
The maximum allowable drain-source voltage must be
higher than the supply voltage. If the output is shorted
to ground or during an overvoltage event, the full supply
voltage will appear across the MOSFET.
The gate drive for the MOSFET is guaranteed to be more
than 10V and less than 18V for those applications with V
higher than 8V. This allows the use of standard threshold
voltage N-channel MOSFETs. For systems with V
than 8V, a logic level MOSFET is required since the gate
drive can be as low as 4.5V.
The SOA of the MOSFET must encompass all fault condi-
tions. In normal operation the pass transistor is fully on,
dissipating very little power. But during either overvoltage
or overcurrent faults, the GATE pin is servoed to regu-
late either the output voltage or the current through the
MOSFET. Large current and high voltage drop across the
MOSFET can coexist in these cases. The SOA curves of
the MOSFET must be considered carefully along with the
selection of the fault timer capacitor.
Transient Stress in the MOSFET
During an overvoltage event, the LT4356-1 drives a series
pass MOSFET to regulate the output voltage at an acceptable
level. The load circuitry may continue operating throughout
this interval, but only at the expense of dissipation in the
MOSFET pass device. MOSFET dissipation or stress is a
function of the input voltage waveform, regulation voltage
and load current. The MOSFET must be sized to survive
this stress.
Most transient event specifi cations use the model shown
in Figure 3. The idealized waveform comprises a linear
(BR)DSS
t
COOL
, the threshold voltage, and the SOA.
=
C
TMR
DS(ON)
2µA
• 0.85V
, the maximum drain-source voltage
CC
43561fd
less
CC