AN1280 Datasheet, PDF (1/8 Pages) STMicroelectronics – ELECTRONIC CIRCUIT BREAKER

AN1280
APPLICATION NOTE
TD230: 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 al-
ways improved reliability after delivery : mainte-
nance 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 Temper-
ature 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 in-
compatible with the host equipment’s require-
ments.
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 conve-
nient solution for any industrial who wants at the
same time :
u immediate, efficient and resettable protec-
tion for his equipment
u versatility regarding different applications
u easy and quick design-in
u low component count
u 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 appli-
cations) 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
of time after which the power MOSFET is defini-
tively switched off, waiting for a reset.
by R. LIOU
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 :
u by switching off the power supplies
u by unplugging and re-plugging the card
(live insertion)
u 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 (I_OC) is detected
through the low ohmic shunt resistors R_{S 1/2} as giv-
en under equation (i) :
u V_{RS 1/2} = I_OC x R_S > 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 I_OC x R_S = 63mV, which means
that the IOC limit is directly given by the choice of
the shunt resistors R_S1/2 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) :
u t_ON = CISS x VTH / 15µA (ii)
where CISS is the input capacitance of the power
MOSFET_1/2 and VTH, the threshold voltage of the
MOSFET (typically 5V).
May 2000
1/8