LT4256 LINER [Linear Technology], LT4256 Datasheet - Page 13

• Manufacturer Part Number: LT4256
• Description: UV, OV and Reverse Supply Protection Controller Low Operating Current: 125?A
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Select Between Two Input Supplies
With the part in shutdown, the V
driven by separate power supplies. The LTC4365 then
automatically drives the GATE pin just below the lower of
the two supplies, thus turning off the external back-to-back
MOSFETs. The application of Figure 11 uses two LTC4365s
to select between two power supplies. Care should be taken
to ensure that only one of the two LTC4365s is enabled
at any given time.
Single MOSFET Application
When reverse V
external N-channel MOSFET is necessary. The applica-
tion circuit of Figure 12 connects the load to V
V
components.
IN
is less than 30V, and uses the minimal set of external
Figure 12. Small Footprint Single MOSFET Application
Protects Against 60V
SEL
24V
V
IN
Figure 11. Selecting One of Two Supplies
IN
R2
2370k
R1
40.2k
V1
V2
protection is not needed, only a single
M1
M1
V
SHDN
V
SHDN
V
SHDN
UV
OV
IN
IN
IN
SI7120DN
SEL
LTC4365
LTC4365
LTC4365
0
1
GATE
GATE
GATE
GND
60V
OUT
V1
V2
FAULT
V
V
V
4365 F11
4365 F12
OUT
OUT
OUT
IN
M2
M2
and V
OV = 30V
+
OUT
OUT
C
100μF
V
OUT
OUT
pins can be
IN
when
Limiting Inrush Current During Turn-On
The LTC4365 turns on the external N-channel MOSFET
with a 20μA current source. The maximum slew rate at
the GATE pin can be reduced by adding a capacitor on
the GATE pin:
Since the MOSFET acts like a source follower, the slew
rate at V
Therefore, inrush current is given by:
For example, a 1A inrush current to a 330μF output
capacitance requires a GATE capacitance of:
The 6.8nF C
Figure 13 limits the inrush current to approximately 1A.
R
turn off characteristics during UV/OV faults, or during
reverse V
frequency oscillations with the external N-channel MOSFET
and related board parasitics.
GATE
I
C
C
Slew Rate =
INRUSH
GATE
GATE
makes sure that C
Figure 13. Limiting Inrush Current with C
OUT
V
=
=
IN
IN
=
20µA • 330µF
20µA • C
connection. R4A and R4B help prevent high
equals the slew rate at GATE.
GATE
C
V
C
IN
I
GATE
INRUSH
OUT
M1
C
20µA
1A
GATE
capacitor in the application circuit of
• 20µA
10Ω
R4A
OUT
LTC4365
GATE
GATE
= 6.6nF
R4B
10Ω
does not affect the fast GATE
M2
V
OUT
4365 F13
+
LTC4365
V
C
330μF
OUT
OUT
GATE
R
5.1k
GATE
C
6.8nF
GATE
13
4365f