LTC2913 datasheet(18/22 Pages) AD

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LTC4368

Rev. B

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Transients During OV Fault

The circuit of Figure 19 is used to illustrate transients

commonly encountered during an overvoltage condition.

The nominal input supply is 48V and it has an overvoltage

threshold of 60V. The parasitic inductance is that of a 1

foot wire (roughly 300nH). Figure 20 shows the wave-

forms during on overvoltage condition at VIN. These tran-

sients depend on the parasitic inductance and resistance

of the wire along with the capacitance at the VIN node.

D1 is an optional power clamp (TVS, TransZorb) recom-

mended for applications where the DC input voltage can

exceed 24V and with large VIN parasitic inductance. No

clamp was used to capture the waveforms of Figure 20. In

order to maintain reverse supply protection, D1 must be

a bidirectional clamp with appropriate voltage and power

ratings.

Layout Considerations

The trace length between the VIN pin and the drain of the

external MOSFET should be minimized, as well as the

trace length between the GATE pin of the LTC4368 and

the gates of the external MOSFETs. The SENSE and VOUT

pins must be connected with traces that tie directly and

solely to the sense resistor.

Place the bypass capacitors at VOUT as close as possible

to the external MOSFET. Use high frequency ceramic

capacitors in addition to bulk capacitors to mitigate hot

swap ringing. Place the high frequency capacitors closest

to the MOSFET. Note that bulk capacitors mitigate ringing

by virtue of their ESR. Ceramic capacitors have low ESR

and can thus ring near their resonant frequency. The trace

length of the GATE pin should be kept as small as pos-

sible, and the number of components connected to the

GATE pin should also be minimized.

The SOA of the external MOSFET might require the board

to have a minimum total area as well as a minimum amount

of trace volume connected to the drain and source pins.

APPLICATIONS INFORMATION

Figure 19. OV Fault with Large VIN Inductance

VIN

48V

FDS3992

100V DUAL

100k

COUT

100µF

2430k

20.5k

RETRY

4368 F19

VOUT

FAULT

LTC4368-2

SENSE

CRETRY

0.22µF

VIN

SHDN

GATE

GND

RSENSE

0.02

CGATE

3.3nF

INRUSH

CONTROL: ~1A

RGATE

22k

1200ms COOLDOWN

AFTER FORWARD

OC FAULT

300nH

(12 INCH WIRE LENGTH)

OV = 60V

Figure 20. Transients During 0V Fault when

No TransZorb (TVS) Is Used

400ns/DIV

VIN

20V/DIV

VOUT

20V/DIV

GATE

IIN

2A/DIV

4368 F20

60V

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