LTC1154

7

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TRUTH TABLE

OPERATION

The Truth Table demonstrates how the LTC1154 receives

inputs and returns status information to the μP. The

ENABLE and input signal from the μP controls the switch

in its normal operating mode, where the rise and fall

time of the gate drive are controlled to limit EMI and RFI

emissions. The shutdown and overcurrent detection cir-

cuitry however, switch the gate off at a much higher rate

to limit the exposure of the MOSFET switch and the load

to dangerous conditions. The status pin remains high as

long as the switch is operating normally, and is driven

low only when a fault condition is detected. Note that the

shutdown pin is edge-sensitive and latches the output off

even if the shutdown pin returns to a low state.

IN

X

L

H

H

H

EN

H

X

L

L

L

SD

X

X

L

L

GATE

L

L

H

L

L

STATUS

H

H

H

L

L

SWITCH OFF

SWITCH OFF

SWITCH ON

SWITCH LATCHED OFF

(OVER CURRENT)

SWITCH LATCHED OFF

(SHUTDOWN)

INPUTS

OUTPUTS

SWITCH

CONDITION

L = LOGIC LOW

H = LOGIC HIGH

X = IRRELEVANT

= EDGE TRIGGERED

The LTC1154 is a single micropower MOSFET driver

with built-in protection, status feedback and gate charge

pump. The LTC1154 consists of the following functional

blocks:

TTL and CMOS Compatible Inputs

The LTC1154 input and shutdown input have been designed

to accommodate a wide range of logic families. Both in-

put thresholds are set at about 1.3V with approximately

100mV of hysteresis.

A low standby current voltage regulator provides continu-

ous bias for the TTL-to-CMOS converter. The TTL-to-CMOS

converter output enables the rest of the circuitry. In this

way the power consumption is kept to a minimum in the

standby mode.

ENABLE Input

The ENABLE input is CMOS compatible and inhibits the

input signal whenever it is held logic high. This input

should be grounded when not in use.

Internal Voltage Regulation

The output of the TTL-to-CMOS converter drives two

regulated supplies which power the low voltage CMOS

logic and analog blocks. The regulator outputs are isolated

from each other so that the noise generated by the charge

pump logic is not coupled into the 100mV reference or

the analog comparator.

Gate Charge Pump

Gate drive for the MOSFET switch is produced by an adap-

tive charge pump circuit which generates a gate voltage

substantially higher than the power supply voltage. The

charge pump capacitors are included on chip and there-

fore no external components are required to generate the

gate drive.

Drain Current Sense

The LTC1154 is configured to sense the current flowing

into the drain of the power MOSFET in a high side ap-

plication. An internal 100mV reference is compared to

the drop across a sense resistor (typically 0.002Ω to

0.10Ω) in series with the drain lead. If the drop across

this resistor exceeds the internal 100mV threshold, the

input latch is reset and the gate is quickly discharged via

a large N-channel transistor.

Controlled Gate Rise and Fall Times

When the input is switched ON and OFF, the gate is charged

by the internal charge pump and discharged in a controlled