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LTC4365

4365f

Select Between Two Input Supplies

With the part in shutdown, the VIN and VOUT pins can be

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.

APPLICATIONS INFORMATION

Figure 11. Selecting One of Two Supplies

VIN

SHDN

4365 F11

VOUT

GATE

LTC4365

VIN

SEL

OUT

SEL

SHDN

VOUT

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

CGATE

Since the MOSFET acts like a source follower, the slew

rate at VOUT equals the slew rate at GATE.

Therefore, inrush current is given by:

INRUSH =

COUT

CGATE

•20µA

For example, a 1A inrush current to a 330μF output

capacitance requires a GATE capacitance of:

CGATE =

20µA • COUT

IINRUSH

CGATE =

20µA • 330µF

= 6.6nF

The 6.8nF CGATE capacitor in the application circuit of

Figure 13 limits the inrush current to approximately 1A.

RGATE makes sure that CGATE does not affect the fast GATE

turn off characteristics during UV/OV faults, or during

reverse VIN connection. R4A and R4B help prevent high

frequency oscillations with the external N-channel MOSFET

and related board parasitics.

VIN

SHDN

OV = 30V

4365 F12

VOUT

FAULT

GATE

VIN

24V

SI7120DN

60V

VOUT

GND

LTC4365

2370k

40.2k

COUT

100μF

Figure 12. Small Footprint Single MOSFET Application

Protects Against 60V

Single MOSFET Application

When reverse VIN protection is not needed, only a single

external N-channel MOSFET is necessary. The applica-

tion circuit of Figure 12 connects the load to VIN when

VIN is less than 30V, and uses the minimal set of external

components.

4365 F13

VIN

VOUT

R4B

10Ω

R4A

10Ω

COUT

330μF

VOUT

GATE

LTC4365

RGATE

5.1k

CGATE

6.8nF

Figure 13. Limiting Inrush Current with CGATE

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