LTC4355

9

4355fe

APPLICATIONS INFORMATION

System Power Supply Failure

The LTC4355 automatically supplies load current from the

system input supply with the higher voltage. If this supply

shorts to ground, reverse current begins to flow through

the pass transistor temporarily and the transistor begins

to turn off. When this reverse current creates –25mV of

voltage drop across the drain and source pins of the pass

transistor, a fast pull-down circuit engages to drive the

gate low faster.

The remaining system power supply delivers the load cur-

rent through the body diode of its pass transistor until the

channel turns on. The LTC4355 ramps the gate up with

20μA, turning on the N-channel MOSFET to reduce the

voltage drop across it.

Input Short-Circuit Faults

The dynamic behavior of an active, ideal diode entering

reverse bias is most accurately characterized by a delay

followed by a period of reverse recovery. During the delay

phase some reverse current is built up, limited by para-

sitic resistances and inductances. During the reverse re-

covery phase, energy stored in the parasitic inductances

is transferred to other elements in the circuit. Current

slew rates during reverse recovery may reach 100A/μs

or higher.

High slew rates coupled with parasitic inductances in se-

ries with the input and output paths may cause potentially

destructive transients to appear at the IN and OUT pins of

the LTC4355 during reverse recovery. A zero impedance

short-circuit directly across an input that is supplying

current is especially troublesome because it permits the

highest possible reverse current to build up during the

delay phase. When the MOSFET finally commutates the

reverse current the LTC4355 IN pin experiences a nega-

tive voltage spike, while the OUT pin spikes in the positive

direction.

To prevent damage to the LTC4355 under conditions of

input short-circuit, protect the IN pins and OUT pin as

shown in Figure 1. The IN pins are protected by clamping

to the GND pin in the negative direction. Protect the OUT

pin with a clamp, such as with a TVS or TransZorb, or with

a local bypass capacitor of at least 10μF. In low voltage

applications the MOSFET’s drain-source breakdown may

Parasitic inductance between the load bypass or the

second supply and the LTC4355 allows a zero impedance

input short to collapse the voltage at the OUT pin, which

up to 30V, bypass the OUT pin with 39μF; above 30V use

at least 100μF. One capacitor serves to guard against OUT

collapse and also protect OUT from voltage spikes.

Figure 1. Reverse Recovery Produces Inductive Spikes at the IN and OUT Pins.

The Polarity of Step Recovery Spikes Is Shown Across Parasitic Inductances

LTC4355

GND

GATE1

GATE2

IN1

OUT

IN2

M1

FDS3672

REVERSE

RECOVERY

CURRENT

M2

FDS3672

SBR1U150SA

INPUT PARASITIC

INDUCTANCE

+

–

OUTPUT PARASITIC

INDUCTANCE

+

–

INPUT PARASITIC

INDUCTANCE

–

+

10μF

SMAT70A

OR

REVERSE RECOVERY CURRENT

4355 F01

SBR1U150SA

INPUT

SHORT