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TD230

ELECTRONIC CIRCUIT BREAKER

STMicroelectronics 

TD230

APPLICATION NOTE

ELECTRONIC CIRCUIT BREAKER

INTRODUCTION

Over current and short circuit protection is a con-

stant concern for today’s engineers. More and

more applications in different segments (Telecom,

Automotive, Industrial, Computer...) require always

improved reliability after delivery : maintenance

costs are an ever more worrying source of

expenses and customers’ dissatisfaction.

Alternatives for short circuit or over current protec-

tions are the fuses and the PTC (Positive Tempera-

ture Coefficient) resistors. The first are a cheap but

destructive solution ; the second are tied to a time

constant due to self heating which is often incom-

patible with the host equipment’s requirements.

In both cases, a coil can be added for an efficient

limitation of current surges, to the detriment of

weight and volume.

None of these solutions is fully satisfactory for a

reliable, immediate and non destructible short cir-

cuit and over current protection.

1. ELECTRONIC CIRCUIT BREAKER

The electronic circuit breaker TD230 is the conven-

ient solution for any industrial who wants at the

same time :

• immediate, efficient and resettable protection

for his equipment

• versatility regarding different applications

• easy and quick design-in

• low component count

• low cost

The electronic circuit breaker TD230 is to be used

with a minimal amount of external and low cost

components to drive one or two N-channel MOS-

FETs (in respectively single or dual supply applica-

tions) used as power switches between the DC

power supplies and the equipments to be

protected.

The TD230 immediately reacts (3µs max. without

load) whenever an over current is detected by

switching off the corresponding MOSFET. Several

automatic restart attempts are made unless the

fault persists over an externally adjustable amount

by R. LIOU

of time after which the power MOSFET is defini-

tively switched off, waiting for a reset.

If the fault is detected on the positive supply, the

definitive shutdown will also disconnect the nega-

tive power supply and set a warning low level on

the Shutdown pin. If the fault is detected on the

negative supply, the definitive shutdown will dis-

connect only the negative power supply,and let the

positive part of the circuit undisturbed.

The whole system can be reset in three ways :

• by switching off the power supplies

• by unplugging and re-plugging the card (live

insertion)

• by setting the INHIBIT pin active during a

short time (allowing remote reset)

2. HOW TO USE THE TD230 ?

The typical configuration of the TD230 - Electronic

Circuit Breaker - in a dual supply topology is shown

in figure 1.

In this configuration, both NMOS 1/2 are used as

power switches which connect the equipments to

the power supplies, thus ensuring low voltage drop

through the ON-resistances (Rdson) of NMOS 1/2.

2.1. Current Limitation

When an over current condition (IOC) is detected

through the low ohmic shunt resistors RS 1/2 as

given under equation (i) :

• VRS 1/2 = IOC x RS > 63mV typ. (i)

the gate of the corresponding MOSFET 1/2 is dis-

charged immediately, thus disconnecting the

board/equipment from the power supply.

Note that the over current condition is given by the

constant product IOC x RS = 63mV, which means

that the IOC limit is directly given by the choice of

the shunt resistors RS1/2 values.

The TD230 automatically makes restart attempts

by slowly recharging the gate of the MOSFET 1/2

with a 15µA typ. current source ensuring thus slow

ramp with the typical time constant before recon-

duction shown in equation (ii) :

• tON = CISS x VTH / 15µA (ii)

6/15

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