6

LTC1155

S

APPLICATI

I FOR ATIO

Gate Charge Pump

Gate drive for the power MOSFET is produced by an

adaptive charge pump circuit that 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 LTC1155 is configured to sense the drain current of

the power MOSFET in high side applications. An internal

100mV reference is compared to the drop across a sense

resistor (typically 0.002

Ω to 0.1Ω) 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 by 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 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.

OPERATIO

Protecting the MOSFET

The MOSFET is protected against destruction by removing

drive from the gate as soon as an overcurrent condition is

detected. Resistive and inductive loads can be protected

with no external time delay. Large capacitive or lamp

loads, however, require that the overcurrent shutdown

function be delayed long enough to start the load but short

enough to ensure the safety of the MOSFET.

Example Calculations

Consider the circuit of Figure 1. A power MOSFET is driven

by one side of an LTC1155 to switch a high inrush current

load. The drain sense resistor is selected to limit the

maximum DC current to 3.3A.

= 0.1/3.3A

= 0.03

Ω

sense pin of the LTC1155 which provides sufficient delay

to start a high inrush load such as large supply capacitors.

In this example circuit, we have selected the IRLZ34

1155 F01

IRLZ34

LOAD

LTC1155

GND

GND

G1

DS1

IN1

0.22

µF

0.03

Ω

270k

Figure 1. Adding an RC Delay

drops 0.1V at 2A and, therefore, dissipates 200mW in

normal operation (no heat sinking required).

If the output is shorted to ground, the current through the

the drain sense resistor and the series resistance be-

tween the power supply and the FET. Series resistance in

the power supply can be substantial and attributed to

many sources including harness wiring, PCB traces,

supply capacitor ESR, transformer resistance or battery

resistance.