stant concern for today’s engineers. More and
If the fault is detected on the positive supply, the more applications in different segments (Telecom, definitive shutdown will also disconnect the nega-Automotive, Industrial, Computer...) require always tive power supply and set a warning low level on improved reliability after delivery : maintenance the Shutdown pin. If the fault is detected on the costs are an ever more worrying source of
negative supply, the definitive shutdown will dis-expenses and customers’ dissatisfaction.
connect only the negative power supply,and let the Alternatives for short circuit or over current protec-positive part of the circuit undisturbed.
tions are the fuses and the PTC (Positive Tempera-The whole system can be reset in three ways :
ture Coefficient) resistors. The first are a cheap but
• by switching off the power supplies
destructive solution ; the second are tied to a time
• by unplugging and re-plugging the card (live
constant due to self heating which is often incom-insertion)
patible with the host equipment’s requirements.
• by setting the INHIBIT pin active during a
In both cases, a coil can be adde d for an efficient short time (allowing remote reset)
limitation of current surges, to the detriment of weight and volume.
2. HOW TO USE THE TD230 ?
None of these solutions is fully satisfactory for a The typical configuration of the TD230 - Electronic reliable, immediate and non destructible short cir-Circuit Breaker - in a dual supply topology is shown cuit and over current protection.
in figure 1.
1. ELECTRONIC CIRCUIT BREAKER
In this configuration, both NMOS 1/2 are used as power switches which connect the equipments to
The electronic circuit breaker TD230 is the conven-the power supplies, thus ensuring low voltage drop ient solution for any industrial who wants at the through the ON-resistances (Rdson) of NMOS 1/2.
same time :
• immediate, efficient and resettable protection 2.1. Current Limitation
for his equipment
When an over current condition (IOC) is detected
• versatility regarding different applications
through the low ohmic shunt resistors RS 1/2 as
• easy and quick design-in
given under equation (i) :
• low component count
• VRS 1/2 = IOC x RS > 63mV typ. (i)
• low cost
the gate of the corresponding MOSFET 1/2 is dis-The electronic circuit breaker TD230 is to be used charged immediately, thus disconnecting the
with a minimal amount of external and low cost
board/equipment from the power supply.
components to drive one or two N-channel MOS-
Note that the over current condition is given by the FETs (in respectively single or dual supply applica-constant product IOC x RS = 63mV, which means
tions) used as power switches between the DC
that the IOC limit is directly given by the choice of power supplies and the equipments to be
the shunt resistors RS1/2 values.
protected.
The TD230 automatically makes restart attempts The TD230 immediately reacts (3µs max. without by slowly recharging the gate of the MOSFET 1/2
load) whenever an over current is detected by
with a 15µA typ. current source ensuring thus slow switching off the corresponding MOSFET. Several ramp with the typical time constant before recon-automatic restart attempts are made unless the
duction shown in equation (ii) :
fault persists over an externally adjustable amount
• tON = CISS x VTH / 15µA (ii)
6/15
TD230
where CISS is the input capacitance of the power Trace A represents the Gate-Source Voltage of the MOSFET1/2 and VTH, the threshold voltage of the Power Mosfet (0 to 13,4V).
MOSFET (typically 5V).
Trace B represents the voltage across the Sense This reconduction time can be extended with an
Resistor (68mΩ) in direct relation with the current external soft start capacitor CSS1/2 as shown in through it (0 to ~1A).
figure 1 CISS will therefore simply be replaced by Note that the first current peak which is due to an CISS + CSS 1/2.
over current is limited only by the reaction time of the TD230.
Figure 1 :
Dual Electronic Circuit Breaker
This off time is tied to the value of the external soft Application
start capacitor CSS 1/2 by equation (iii) :
• tOFF = RDSON x CSS (iii)
While in current limitation mode, the NMOS1/2 dis-RS 1
sipates low power due to the fact that the ON/OFF
Vcc+
cycle time rate is very low.
P Vcc
R EF1
1
16
Note that the higher the value of C
LBOO S T
SS1/2 are, the
LBOOS T
GC1
NMos
2
15
CS S 1
more the NMOS
to BOARD
1/2 will stay in linear mode during
CB OOS T
S E NSP
3
14
CBO OS T
current limitation.
O S C GND
INHIBIT
4
13
Note that at Power ON, or in the case of live
C ONTRO L
5
P M1
S HUTDOWN
12
insertion, the inrush current is automatically limited CTRIP 1
from BO ARD
6
GND
S ENSN
11
CTRIP 2
thanks to the slow gate charge of the MOSFET
NMOS
GND
P M2
7
GC 2
10
which switches ON softly due to the time constant CS S 2
NVcc
R EF2
9
8
given in equation (ii).
Vcc-
RS 2
2.2. Fault Time Limitation