Linear Technology DC852A - LTC4354 Negative Voltage Diode-OR Controller and Monitor DC852A DC852A Data Sheet

Product codes

DC852A

LTC4354

4354fc

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

through an external current limiting resistor (R

). An

internal shunt regulator clamps the voltage at V

pin to

11V. A 1µF decoupling capacitor to V

is recommended.

It also provides a soft-start to the part.
R

should be chosen to accommodate the maximum

supply current requirement of 2mA at the expected input

operating voltage.

≤

IN(MIN)

− V

Z(MAX)

)

CC(MAX)

The power dissipation of the resistor is calculated at the

maximum DC input voltage:

IN(MAX)

− V

CC(MIN)

)

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

on-resistance R

DS(ON)

, the maximum drain-source voltage

DSS

, and the threshold voltage.

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 external zener

diode can be used to clamp the potential at the V

to as low as 4.5V if the gate to source rated breakdown

voltage is less than 12V.
The maximum allowable drain-source voltage, V

(BR)DSS,

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

shorted, the full supply voltage will appear across the

MOSFETs.

applicaTions inForMaTion

Figure 1. Method of Protecting the DA and DB Pins from

Negative Inputs. One Channel Shown

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 260mV fault threshold. The

DS(ON)

should be small enough to conduct the maximum

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

the MOSFET’s power rating at the maximum load current

• R

DS(ON)

Fault Conditions
LTC4354 monitors fault conditions and turns on an LED

or opto-coupler to indicate a fault. When the voltage drop

across the pass transistor is higher than the 260mV fault

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

and allows the current to flow through the LED or opto-

coupler. Conditions that cause high voltage across the pass

transistor include: short in the load circuitry, excessive

load current, FET open while conducting current, and FET

short on the channel with the higher supply voltage. The

fault threshold is internally set to 260mV.
In the event of FET open on the channel with the more

negative supply voltage, if the voltage difference is high

enough, the substrate diode on the DA or DB pins will

forward bias. The current flowing out of the pins must

be limited to a safe level (<1mA) to prevent device latch

up. Schottky diodes can be used to clamp the voltage at

the DA and DB pins, as shown in Figure 1.

4354 F01

LTC4354

MMBD2836LT1