LTC4354

7

4354f

APPLICATIO S I FOR ATIO

Input Power Supply

The power supply for the device is derived from –48_RTN

It also provides a soft-start to the part.

supply current requirement of 2mA at the expected input

operating voltage.

R

VV

I

IN

IN MIN

Z MAX

CC MAX

≤

−

()

()

(

)

()

The power dissipation of the resistor is calculated at the

maximum DC input voltage:

P

VV

R

IN MAX

CC MIN

IN

=

−

()

()

(

)

2

If the power dissipation is too high for a single resistor, use

multiple low power resistors in series instead of a single

high power component.

MOSFET SELECTION

The LTC4354 drives N-channel MOSFETs to conduct the

load current. The important features of the MOSFETs are

The gate drive for the MOSFET is guaranteed to be more

than 10V and less than 12V. This allows the use of

standard threshold voltage N-channel MOSFETs. An ex-

ternal zener diode can be used to clamp the potential at the

breakdown voltage is less than 12V.

must be higher than the supply voltages. If the inputs are

shorted, the full supply voltage will appear across the

MOSFETs.

The LTC4354 tries to servo the voltage drop across the

MOSFET to 30mV in the forward direction by controlling

the gate voltage and sends out a fault signal when the

voltage drop exceeds the 250mV fault threshold. The

load current while not triggering a fault, and to stay within

the MOSFET’s power rating at the maximum load current

Fault Conditions

LTC4354 monitors fault conditions and turns on an LED or

optocoupler to indicate a fault. When the voltage drop

across the pass transistor is higher than the 250mV fault

threshold, the internal pull-down at the FAULT pin turns off

and allows the current to flow through the LED or

optocoupler. Conditions that cause high voltage across

the pass transistor include: short in the load circuitry,

excessive load current, FET open while conducting cur-

rent, and FET short on the channel with the higher supply

voltage. The fault threshold is internally set to 250mV.