LTC1255I View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1255I

Dual 24V High-Side MOSFET Driver

Linear Technology 

LTC1255

PI FU CTIO S

Input Pin

The LTC1255 input pin is active high and activates all of

the protection and charge pump circuitry when switched

ON. The LTC1255 logic and shutdown inputs are high

impedance CMOS gates with ESD protection diodes to

ground and supply and therefore should not be forced

beyond the power supply rails. The input pin should be

held low during the application of power to properly set

the input latch.

Gate Drive Pin

The gate drive pin is either driven to ground when the

switch is turned OFF or driven above the supply rail

when the switch is turned ON. This pin is of relatively

high impedance when driven above the rail (the equiva-

lent of a few hundred kΩ). Care should be taken to

minimize any loading of this pin by parasitic resistance

to ground or supply.

Supply Pin

The supply pin of the LTC1255 serves two vital pur-

poses. The first is obvious; it powers the input, gate

drive, regulation and protection circuitry. The second

purpose is less obvious; it provides a Kelvin connection

to the top of the drain sense resistor for the internal

100mV reference.

The supply pin of the LTC1255 should never be forced

below ground as this may result in permanent damage

to the device. A 100Ω resistor should be inserted in

series with the ground pin if negative supply voltage

transients are anticipated.

The LTC1255 is designed to be continuously powered

so that the gate of the MOSFET is actively driven at all

times. If it is necessary to remove power from the

supply pin and then reapply it, the input pin should be

cycled (low to high) a few milliseconds after the power

is reapplied to reset the input latch and protection

circuitry. Also, the input pin should be isolated from the

controlling logic by a 10k resistor if there is a possibility

that the input pin will be held high after the supply has

been removed.

Drain Sense Pin

The drain sense pin is compared against the supply pin

voltage. If the voltage at this pin is more than 100mV

below the supply pin, the input latch will be reset and

the MOSFET gate will be quickly discharged. Cycle the

input to reset the short-circuit latch and turn the MOSFET

back on.

This pin is also a high impedance CMOS gate with ESD

protection and therefore should not be forced outside

of the power supply rails. To defeat the overcurrent

protection, short the drain sense pin to the supply pin.

Some loads, such as large supply capacitors, lamps or

motors require high in-rush currents. An RC time delay

can be added between the sense resistor and the drain

sense pin to ensure that the drain sense circuitry does

not false trigger during startup. This time constant can

be set from a few microseconds to many seconds.

However, very long delays may put the MOSFET at risk

of being destroyed by a short-circuit condition (see

Applications Information section).

U

OPERATIO

The LTC1255 is a dual 24V MOSFET driver with built-in

protection and gate charge pump. The LTC1255 consists

of the following functional blocks:

TTL and CMOS Compatible Inputs and Latches

The LTC1255 inputs have been designed to accommo-

date a wide range of logic families. Both input thresh-

olds are set at about 1.3V with approximately 100mV of

hysteresis. A low standby current regulator provides

continuous bias for the TTL-to-CMOS converter.

The input/protection latch should be set after initial

power-up, or after reapplication of power, by cycling

the input low to high.

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