Datasheet
LT4356-1/LT4356-2
15
4356fb
For more information www.linear.com/LT4356-1
Figure 5
Figure 6. Auxiliary LDO Output with Optional Current Limit
C1
R3
4356 F05
LT4356S
GATE
Q1
R1
D1
IN4148W
R
LIM
*4.7Ω
D1*
BAV99
2N2905A OR
BCP53
INPUT
2.5V OUTPUT
≈ 150mA MAX
* OPTIONAL FOR
CURRENT LIMIT
LT4356S
A
OUT
IN
+
11
12
R6
100k
4356 F06
R4
249k
47nF
R5
249k
10µF
V
OUT
= 1.25
R4 + R5
R5
I
LIM
≈
0.7
R
LIM
ApplicAtions inForMAtion
The LTC4356 does not need extra compensation compo-
nents at the GATE pin for stability during an overvoltage or
overcurrent event. With transient input voltage step 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.
Reverse Input Protection
A blocking diode is commonly employed when reverse
input potential is possible, such as in automotive applica
-
tions. 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 LT4356 is designed to withstand reverse voltage
without damage to itself or the load. The V
CC
, SNS, and
SHDN pins can withstand up to 60V of DC voltage below
the GND potential. Back-to-back MOSFETs must be used
to eliminate the current path through their body diodes
(Figure 7). Figure 8 shows the approach with a P-Channel
MOSFET in place of Q2.
Figure 7. Overvoltage Regulator with N-channel MOSFET
Reverse Input Protection
C
TMR
0.1µF
R
SNS
10mΩ
Q1
IRLR2908
Q2
IRLR2908
V
IN
12V
V
OUT
12V, 3A
CLAMPED
AT 16V
4356 F07
LT4356S
GND TMR
10 1
OUTSNS
35
SHDN
7
A
OUT
11
IN
+
12
V
CC
6
EN
FLT
FB
9
8
2
D2*
SMAJ58CA
R2
4.99k
R1
59k
GATE
4
R7
10k
R5
1M
Q3
2N3904
D1
1N4148
R3
10Ω
R4
10Ω
*DIODES INC.
Auxiliary Amplifier
An uncommitted amplifier is included in the LT4356 to
provide flexibility in the system design. With the negative
input connected internally to the 1.25V reference, the am
-
plifier can be connected as a level detect comparator with
external hysteresis. The open collector output pin, A
OUT
,
is capable of driving an opto or LED. It can also interface
with the system via a pull-up resistor to a supply voltage
up to 80V. Another use is to implement undervoltage
lockout, as shown in the typical application “Overvoltage
Regulator with Undervoltage Lockout.” In shutdown, the
auxiliary amplifier turns off in the LT4356-1 but continues
operating in the LT4356-2.
The amplifier can also be configured as a low dropout
linear regulator controller. With an external PNP transistor,
such as 2N2905A, it can supply up to 100mA of current
with only a few hundred mV of dropout voltage. Current
limit can be easily included by adding two diodes and one
resistor (Figure 6).
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