LTC2913 datasheet(15/22 Pages) AD

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LTC4368

Rev. B

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Figure 11. Reverse VIN Protection Circuits

VIN

–40V

LOAD

Si7942 100V DUAL

COUT

100µF

VOUT

LTC4368

SENSE

GATE

CGATE

3.3nF

INRUSH

CONTROL

RGATE

22k

VIN

CHOTSWAP

(OPTIONAL

4.4nF)

GND

REVERSE VIN

COMPARATOR

CLOSES SWITCH

WHEN VIN IS NEGATIVE

4368 F11

Reverse VIN Protection

The LTC4368’s rugged and hot swappable VIN helps pro-

tect the more sensitive circuits at the output load. If the

input supply is plugged in backwards, or a negative sup-

ply is inadvertently connected, the LTC4368 prevents this

negative voltage from passing to the output load.

As shown in Figure 11, 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 current

can flow from VOUT to VIN. Note that the voltage rating of

M2 must withstand the reverse voltage excursion at VIN.

parasitic inductance of the VIN and GATE connections, the

voltage at the VIN and GATE pins ring significantly below

–20V. Therefore, hot swapping a negative input voltage

more negative than –20V should not be performed with-

out additional overshoot mitigation techniques in place.

The front page application was used to generate the wave-

forms of Figure 12.

APPLICATIONS INFORMATION

The speed of the LTC4368 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.

The trace at VOUT, on the other hand, does not respond

to the negative voltage at VIN, demonstrating the desired

reverse supply protection. The waveforms of Figure 12

were captured using a 40V dual N-channel MOSFET, a

10µF ceramic output capacitor and no load current on

VOUT.

Hot Swap VIN Protection

The VIN input of the LTC4368 can be live inserted or hot

swapped into a backplane with minor disturbance to the

VIN supply. The idea is to keep the parasitic capacitances

of the external MOSFETs (CGD) from coupling onto the

GATE pin and enhancing the MOSFETs. To improve

positive VIN hot swap capability (without jeopardizing

reverse polarity protection), place CHOTSWAP across the

gate and source terminals of the back-to-back MOSFETs.

Figure 13 illustrates the waveforms that result when

the VIN of the front page application is hot plugged to

+48V. The top trace is VIN. The bottom two traces are the

MOSFETs gate and source terminals. Note that the bottom

two traces ring together and thus keep the MOSFETs off

To avoid large currents when the reverse voltage is hot

plugged, set RGATE to 22k. To further improve reverse

hot swap performance, place the optional CHOTSWAP ≥

4.4nF capacitor across the gate and source terminals of

the external MOSFETs.

Figure 12 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

Figure 12. Hot Swapping VIN to –20V

VOUT

GATE

VIN

–20V

200ns/DIV

LTC4368 F12

5V/DIV

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