Datasheet

LTC4365
14
4365fa
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the two supplies, thus turning off the external back-to-back
MOSFETs. The application of Figure 12 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.
applicaTions inForMaTion
Figure 12. Selecting One of Tw o Supplies
V
IN
V2
SHDN
4365 F12
V
OUT
GATE
LTC4365
V
IN
V1
SEL
0
1
OUT
V1
V2
OUT
M2M1
M2M1
SEL
SHDN
V
OUT
GATE
LTC4365
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:
Slew Rate =
20µA
C
GATE
Since the MOSFET acts like a source follower, the slew
rate at V
OUT
equals the slew rate at GATE.
Therefore, inrush current is given by:
I
INRUSH
=
C
OUT
C
GATE
• 20µA
For example, a 1A inrush current to a 330µF output
capacitance requires a GATE capacitance of:
C
GATE
=
20µA • C
OUT
I
INRUSH
C
GATE
=
20µA • 330µF
1A
= 6.6nF
The 6.8nF C
GATE
capacitor in the application circuit of
Figure 14 limits the
inrush current to approximately 1A.
R
GATE
makes sure that C
GATE
does not affect the fast GATE
turn off characteristics during UV/OV faults, or during
reverse V
IN
connection. R4A and R4B help prevent high
frequency oscillations with the external N-channel MOSFET
and related board parasitics.
V
IN
UV
OV
SHDN
OV = 30V
4365 F13
V
OUT
FAULT
GATE
V
IN
24V
SI7120DN
60V
V
OUT
GND
LTC4365
R2
2370k
R1
40.2k
C
OUT
100µF
+
R5
100k
Figure 13. Small Footprint Single MOSFET Application
Protects Against 60V
4365 F14
V
IN
V
IN
V
OUT
R4B
10Ω
R4A
10Ω
C
OUT
330µF
V
OUT
GATE
LTC4365
R
GATE
5.1k
C
GATE
6.8nF
+
M2M1
Figure 14. Limiting Inrush Current with C
GATE
Single MOSFET Application
When reverse V
IN
protection is not needed, only a single
external N-channel MOSFET is necessary. The applica-
tion cir
cuit of Figure 13 connects the load to V
IN
when
V
IN
is less than 30V, and uses the minimal set of external
components.