10

LTC1473

will minimize the heat dissipated in the switches while

increasing the overall system efficiency. Higher switch

resistances can be tolerated in some systems with lower

current requirements, but care should be taken to ensure

that the power dissipated in the switches is never allowed

to rise above the manufacturers’ recommended level.

A small valued sense resistor (current shunt) is used by

the two switch pair drivers to measure and limit the inrush

or short-circuit current flowing through the conducting

switch pair.

The inrush current limit should be set at approximately 2

×

or 3

× the maximum required output current. For example,

if the maximum current required by the DC/DC converter

is 2A, an inrush current limit of 6A is set by selecting a

0.033

mula:

Note that the voltage drop across the resistor in this

example is only 66mV under normal operating conditions.

Therefore, the power dissipated in the resistor is ex-

tremely small (132mW), and a small 1/4W surface mount

resistor can be used in this application (the resistor will

tolerate the higher power dissipation during current limit

for the duration of the fault time-out). A number of small

valued surface mount resistors are available that have

been specifically designed for high efficiency current

sensing applications.

duration the MOSFET switches are allowed to be in con-

tinuous current limit.

In the event of a fault condition, the MOSFET switch is

driven into current limit by the inrush current limit loop.

The MOSFET switch operating in current limit is in a high

dissipation mode and can fail catastrophically if not

promptly terminated.

The fault time delay is programmed with an external

capacitor between the TIMER pin and GND. At the instant

the MOSFET switch enters current limit, a 5.5

µA current

latch is set and the MOSFET switch is turned off. To reset

the latch, the logic input of the MOSFET gate driver is

deselected.

The fault time delay should be programmed as large as

possible, at least 3

× to 5× the maximum switching transi-

tion period, to avoid prematurely tripping the protection

circuit. Conversely, for the protection circuit to be effec-

tive, the fault time delay must be within the safe operating

area of the MOSFET switches, as stated in the

manufacturer’s data sheet.

The maximum switching transition period happens during

a cold start, when a fully charged battery is connected to

an unpowered system. The inrush current charging the

system supply capacitor to the battery voltage determines

the switching transition period.

Assume the maximum battery voltage is 20V, the system

supply capacitor is 68

µF, the inrush current limit is 6A and

the maximum current required by the DC/DC converter is

2A. Then, the maximum switching transition period is

calculated using the following formula:

= 340

µs

(20)(68

µF)

6A – 2A

Multiplying 3 by 340

µs gives 1.02ms, the minimum fault

delay time. Make sure this delay time does not fall outside

of the safe operating area of the MOSFET switch dissipat-

be calculated using the following formula:

= 4700pF

5.5

µA

1.20V

)

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