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LTC4365

4365f

APPLICATIONS INFORMATION

The resistor values can then be solved:

1. R1+ R2 =

3mV

10nA

= 300k

2. R3 = 2•

3mV

10nA

• (3.5V – 0.5V) =1.8M

The closest 1% value: R3 = 1.82M:

3. R1 =

300k +1.82M

2 • 18V

= 58.9k

The closest 1% value: R1 = 59k:

R2 = 300k – 59k = 241k

The closest 1% value: R2 = 243k

Therefore: OV = 17.93V, UV = 3.51V.

Reverse VIN Protection

The LTC4365’s rugged and hot-swappable VIN input helps

protect the more sensitive circuits at the output load. If

the input supply is plugged in backwards, or a negative

supply is inadvertently connected, the LTC4365 prevents

this negative voltage from passing to the output load.

The LTC4365 employs a novel, high speed reverse sup-

ply voltage monitor. When the negative VIN voltage is

detected, an internal switch connects the gates of the

external back-to-back N-channel MOSFETs to the nega-

tive input supply.

As shown in Figure 6, external back-to-back N-channel

MOSFETs are required for reverse supply protection. When

VIN goes negative, the reverse VIN comparator closes the

internal switch, which in turn connects the gates of the

external MOSFETs to the negative VIN voltage. The body

diode (D1) of M1 turns on, but the body diode (D2) of M2

remains in reverse blocking mode. This means that the

common source connection of M1 and M2 remains about

a diode drop higher than VIN. Since the gate voltage of

M2 is shorted to VIN, M2 will be turned off and no cur-

rent can flow from VIN to the load at VOUT. Note that the

voltage rating of M2 must withstand the reverse voltage

excursion at VIN.

Figure 7 illustrates the waveforms that result when VIN

is hot plugged to –20V. VIN, GATE and VOUT start out at

ground just before the connection is made. Due to the

parasitic inductance of the VIN and GATE connections, the

voltage at the VIN and GATE pins ring significantly below

–20V. Therefore, a 40V N-channel MOSFET was selected

to survive the overshoot.

The speed of the LTC4365 reverse protection circuits is

evident by how closely the GATE pin follows VIN during

the negative transients. The two waveforms are almost

indistinguishable on the scale shown.

Figure 6. Reverse VIN Protection Circuits

VIN

4365 F06

VOUT

GATE

VIN = –40V

REVERSE VIN

COMPARATOR

CLOSES SWITCH

WHEN VIN IS NEGATIVE

GND

LTC4365

–

TO LOAD

COUT

Figure 7. Hot Swapping VIN to –20V

–20V

5V/DIV

GND

4365 F07

500ns/DIV

GATE

VOUT

VIN

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