6

LTC1255

OPERATIO

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 power MOSFET is produced by an

adaptive charge pump circuit which generates a gate

voltage substantially higher than the power supply volt-

age. The charge pump capacitors are included on-chip and

therefore no external components are required to generate

the gate drive. The charge pump is designed to drive a 12V

Zener diode clamp connected across the gate and source

of the MOSFET switch.

(One Channel)

BLOCK DIAGRA

Drain Current Sense

The LTC1255 is configured to sense the current flowing

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

tion. An internal 100mV reference is compared to the drop

across a sense resistor (typically 0.002

Ω 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 relatively

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 manner. The charge and discharge rates have

been set to minimize RFI and EMI emissions in normal

operation. If a short circuit or current overload condition

is encountered, the gate is discharged very quickly (typi-

cally a few microseconds) by a large N-channel transistor.

100mV

REFERENCE

10

µs

DELAY

LOW STANDBY

CURRENT

REGULATOR

TTL-TO-CMOS

CONVERTER

VOLTAGE

REGULATOR

ANALOG

DIGITAL

INPUT

LATCH

R

S

ONE

SHOT

OSCILLATOR

AND CHARGE

PUMP

GATE CHARGE

AND DISCHARGE

CONTROL LOGIC

FAST/SLOW

GATE CHARGE

LOGIC

INPUT

GATE

DRAIN

SENSE

ANALOG SECTION

COMP

GND

LTC1255 • BD