Linear Technology LT4363 Datasheet page 15

applicaTions inForMaTion
Limiting Inrush Current and GATE Pin Compensation
The LT4363 limits the inrush current to any load capacitance
by controlling the GATE pin voltage slew rate. An external
capacitor can be connected from GATE to ground to reduce
the inrush current at the expense of slower turn-off time.
The gate capacitor is set at:
I
GATE(UP)
C1= •C
L
I
INRUSH
The LT4363 does not need extra compensation compo-
nents at the GATE pin for stability during an overvoltage or
overcurrent event. With transient input voltage slew rates
faster than 5V/µs, a gate capacitor, C1, to ground is needed
to prevent self enhancement of the N-channel MOSFET.
The extra gate capacitance slows down the turn off time
during fault conditions and may allow excessive current
during an output short event. An extra resistor, R1, in series
with the gate capacitor can improve the turn off time. A
diode, D1, should be placed across R1 with the cathode
connected to C1 as shown in Figure 5.
Q1
R3
GATE
LT4363
Figure 5. External GATE network
Undervoltage/Overvoltage Comparators
The LT4363 has both undervoltage and overvoltage com-
parators that can be used to sense the input supply volt-
age. When the voltage at the UV pin is below the 1.275V
threshold, the GATE pin is held low to keep the external
MOSFET off. The supply voltage at the V
at least 4V for the UV comparator to function.
The overvoltage comparator prevents the LT4363-2 from
restarting if the voltage at the OV pin is above the 1.275V
D1
1N4148W
R1
C1
4363 F05
pin should be
CC
For more information
threshold during a fault. The pass transistor is not allowed
to turn back on even after the cool down period has finished.
This prevents the pass transistor from cycling between ON
and OFF states when the input voltage stays at an elevated
level for a long period of time, reducing the stress on the
N-channel MOSFET. For the latch-off version, LT4363-1,
the overvoltage comparator function is not available.
Reverse Input Protection
A blocking diode is commonly employed to protect the
load when reverse input is possible, such as in automo-
tive applications. This diode causes extra power loss,
generates heat, and reduces the available supply voltage
range. During cold crank, the extra voltage drop across
the diode is particularly undesirable.
The LT4363 is designed to withstand reverse voltage with-
out damage to itself. The V
withstand up to 60V of DC voltage below the GND potential.
Back-to-back MOSFETs must be used to block the current
path through Q1's body diode (Figure 6). Figure 7 shows
the approach with a P-channel MOSFET in place of Q2.
Q2
IRLR2908
V
IN
12V
D1*
SMAJ58CA
Q3
2N3904
D2
R7
1N4148
10k
5
V
CC
6
SHDN
8
UV
7
OV
*DIODES INC.
Figure 6. Overvoltage Regulator with N-channel MOSFET
Reverse Input Protection
www.linear.com/LT4363
LT4363
, SHDN, UV, and OV pins can
CC
R
Q1
SNS
10m
IRLR2908
R4 R5 R3
10 1M 10
R1
57.6k
C1
47nF
4 3 2
GATE SNS OUT
1
FB
R2
4.99k
LT4363DE-2
11
ENOUT
10
FLT
GND TMR
9 12 4363 F06
C
TMR
0.1µF
V
OUT
12V, 3A
CLAMPED
AT 16V
4363fb
15