LTC4355 LINER [Linear Technology], LTC4355 Datasheet - Page 9

• Manufacturer Part Number: LTC4355
• Description: Positive High Voltage Ideal Diode-OR with Input Supply and Fuse Monitors
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
System Power Supply Failure
The LTC4355 automatically supplies load current from
the system input supply with the higher voltage. If this
supply shorts to ground, reverse current begins to flow
through the pass transistor temporarily and the transis-
tor begins to turn off. When this reverse current creates
–25mV of voltage drop across the drain and source pins
of the pass transistor, a fast pulldown circuit engages to
drive the gate low faster.
The remaining system power supply delivers the load cur-
rent through the body diode of its pass transistor until the
channel turns on. The LTC4355 ramps the gate up with
20µA, turning on the N-channel MOSFET to reduce the
voltage drop across it.
When the capacitances at the inputs and output are very
small, large changes in current can cause inductive tran-
sients that exceed the 100V Absolute Maximum Ratings
of the pins. A surge suppressor (TransZorb) at the output
will minimize this ringing.
Loop Stability
The servo loop is compensated by the parasitic capaci-
tance of the power N-channel MOSFET. No further com-
pensation components are normally required. In the case
when a MOSFET with less than 1000pF gate capacitance
is chosen, a 1000pF compensation capacitor connected
across the gate and source pins might be required.
V
V
IN1
IN2
= +48V
= +48V
GND
12.7k
340k
R2
R1
12.7k
340k
R3
R4
Figure 1. 36V to 72V/5A Design Example
F1
7A
F2
7A
MON2
MON1
SET
IN1
FDS3672
GATE1
M1
LTC4355
GND
IN2
FDS3672
GATE2
PANASONIC LN1351C
M2
Design Example
The following design example demonstrates the calcula-
tions involved for selecting components in a 36V to 72V
system with 5A maximum load current (see Figure 1).
First, choose the N-channel MOSFET. The 100V, FDS3672
in the SO-8 package with R
good solution. The maximum voltage drop across it is:
The maximum power dissipation in the MOSFET is a mere:
Next, select the resistive dividers that guarantee the
PWRFLT pins willl not assert when the input supplies are
above 36V. The maximum V
maximum I
R1 = 12.7kW. Then,
Use I
PWRFLT2
FUSEFLT1
FUSEFLT2
PWRFLT1
VDSFLT
DV = 5A • 22mW = 110mV
P = 5A • 110mV = 0.55W
OUT
I
R
=
R
GREEN LEDs
2
2
R2
1
=
=
2 2 7
to choose R2.
.
36
1 245
V
R MIN
k
MONx TH
.
MONx(IN)
W −
1
V
100
(
D1
(
−
33k
R5
V
1 245
(
1
µ
.
%)
D2
)
33k
R6
A
)
is 1µA. Choose a 1% tolerance resistor
+
+
D3
V
33k
I
R7
MONx TH MAX
1
µ
=
D4
33k
A
4355 F01
R8
348
DS(ON)
=
(
MONx(TH)
D5
33k
100
R9
k
)(
W
TO
LOAD
= 22mW(max) offers a
µ
A
)
is 1.245V and the
LTC4355
4355fa