AN1280
APPLICATION NOTE
TD230: ELECTRONIC CIRCUIT BREAKER
by R. LIOU
O
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.
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
.
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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
AN1280 - APPLICATION NOTE
This reconduction time can be extended with an
external soft start capacitor C
SS1/2
as shown in
figure 1
C
ISS
will therefore simply be replaced by
C
ISS
+ C
SS 1/2
.
Figure 1 :
Dual Electronic Circuit Breaker
Application
Trace A
represents the Gate-Source Voltage of
the Power Mosfet (0 to 13,4V).
Trace B
represents the voltage across the Sense
Resistor (68mΩ) in direct relation with the current
through it (0 to ~1A).
Note that the first current peak which is due to an
over current is limited only by the reaction time of
the
TD230.
This off time is tied to the value of the external soft
start capacitor C
SS 1/2
by equation (iii) :
u
t
OFF
= R
DSON
x C
SS
(iii)
While in current limitation mode, the NMOS
1/2
dis-
sipates low power due to the fact that the ON/OFF
cycle time rate is very low.
Note that the higher the value of C
SS1/2
are, the
more the NMOS
1/2
will stay in linear mode during
current limitation.
Note that at Power ON, or in the case of live inser-
tion, the inrush current is automatically limited
thanks to the slow gate charge of the MOSFET
which switches ON softly due to the time constant
given in equation (ii).
2.2. Fault Time Limitation
If the fault (over current condition) still remains af-
ter the reconduction state of the MOSFET
1/2
has
been reached, the current through NMOS
1/2
will
overpass the limitation given by equation (i), and
the NMOS
1/2
will immediately be switched off
again.
Figure 2
shows the current limitation which is op-
erated on every restart attempt.
Figure 2 :
TD230 as Current Limitor
The repetitive switching off of the MOSFET will
come to an end under two conditions :
u
either the fault has disappeared, and the
current through the shunt resistors R
S 1/2
has come back to its nominal value : the
system keeps running normally.
External line defaults (lightning, line breakage,
etc...) are usual causes for such temporary over
currents.
u
either the repetitive switching off has lasted
over an externally adjustable time and the
TD230
has definitively switched off the cor-
responding NMOS : the system waits to be
reset.
Equipment faults (component short circuit, over
heat, etc ...) are usual causes for lasting over cur-
rents.
This fault time supervision is done by the compar-
ison of the output voltage to 75% of the nominal
supply voltage. As soon as the output voltage is
detected under 0.75xVcc(+/-), the corresponding
external capacitors C
TRIP1/2
is charged by a fixed
current source I
P/N2
- I
P/N3
(3µA). When the voltage
across C
TRIP1/2
reaches 1.20V, the corresponding
NMOS is definitively switched off and the SHUT-
DOWN pin is active low.
RS1
Vcc+
LBOOST
1
2
PVcc
REF1
16
15
14
13
12
11
10
9
CSS2
from BOARD
NMOS
CONTROL
LBOOST
GC1
NMos
CSS1
to BOARD
CBOOST
3
4
5
CBOOST
OSCGND
SENSP
INHIBIT
PM1
SHUTDOWN
CTRIP1
CTRIP2
GND
6
7
GND
SENSN
PM2
NVcc
GC2
REF2
Vcc-
8
RS2
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2/8
AN1280 - APPLICATION NOTE
To avoid cumulative charging of the protection ca-
pacitors C
TRIP 1/2
in case of successive overcurrent
conditions, the capacitors C
TRIP 1/2
are constantly
discharged by another fixed current source IP/N3
which value is a fourth of I
P/N2
(1µA).
Figure 3 :
Fault Time Limitation
Figure 4 :
Step Up Converter External
Components
Rsense
Lboost
Sense
Cboost
Step Up
Driver
TD230
MOS
The principle of this inductive step-up converter is
to pump charges in the tank capacitor C
BOOST
fol-
lowing the equation (v) :
Figure 5 :
Internal Step Up Schematic
Trace 1
represents the C
BOOST
Voltage (0 to
5+13,4 = 18,4V)
Trace 2
represents the C
TRIP1
Voltage.
The value of the capacitors C
TRIP 1/2
should be cho-
sen in relation with the required protection time as
indicated in equation (iv) :
u
C
TRIP1/2
= (I
P/N2
- I
P/N3
) x t
PROTECT1/2
/ VS
PN/3
(iv)
where t
PROTECT 1/2
is the time defined by the user be-
fore a definitive resettable shutdown of MOS-
FET
1/2
.
Equation (iv) can be translated to :
u
C
TRIP 1/2
= t
PROTECT 1/2
x 3µA / 1.20V (iv)
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Note that the positive power supply disjonction
leads to the negative power supply disjonction,
whereas the opposite is not true.
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Lboost
Cboost
Regulation
TD230
u
V(C
BOOST
) = V
CC+
+ 13.4V typ (v)
Charges are pumped by means of an oscillator
commanded switch, and stored in the C
BOOST
tank
capacitor through a diode as shown on
figure 5.
When the voltage across C
BOOST
reaches
V
CC
+
+13.4V typ, the oscillator is stopped. This cre-
ates a ripple voltage with an amplitude of 0.2V.
Note that the min and max values of V(C
BOOST
)
comprised between V
CC
+
+10V and V
CC
+
+15V al-
ready take the ripple voltage into account.
2.3. Step-Up Converter
To ensure proper voltage on the gate of the posi-
tive supply NMOS1 (V
GS
= 13.4V typ), the
TD230
integrates a step-up converter which is to be
boosted with two small low cost external compo-
nents : an inductor L
BOOST
and a capacitor C
BOOST
,
as shown in
figure 4.
3/8
AN1280 - APPLICATION NOTE
Proper operation of this step-up converter is guar-
anteed at as low as 2.7V with a rise time (0 to 90%
of V(C
BOOST
)) in the range of 700µs at 2.7V which
is the worst case. At 5V, the rise time of V(C
BOOST
)
is 250µs typ. The C
BOOST
voltage wave form at
power ON under 5V supply voltage is shown on
figure 6.
Figure 6 :
Step Up Converter Rise Time
2.4. Single Supply Breaker Application
The
TD230
is perfectly suited to fit in single sup-
plied applications (ex 0-5V), and can drive only
one power MOSFET used as high side power
switch.
Figure 7
shows how
TD230
can be used as a sin-
gle circuit breaker with the same performances.
Figure 7 :
Single Electronic Circuit Breaker
Application
RS1
Vcc+
LBOOST
1
2
PVcc
REF1
16
15
14
13
12
11
NMos
CSS1
to BOARD
LBOOST
GC1
CBOOST
3
4
5
CBOOST
OSCGND
SENSP
INHIBIT
CONTROL
PM1
SHUTDOWN
CTRIP1
6
7
GND
PM2
NVcc
GND
Trace 1
represents the power supply voltage (0 to
5V).
Trace 2
represents the C
BOOST
Voltage at power
ON (0 to 5+13,4 = 18,4V).
Table (a)
summerizes the recommended values
of the C
BOOST
and L
BOOST
to ensure optimized gate
charge and low ripple voltage with their corre-
sponding maximum current surge (I
PK
) and nomi-
nal consumption (I
CC
) of the
TD230
for the most
common power supply values. For each power
supply value is also given the recommended value
of a bypass capacitor (C
BY
) on the power sup-
plies.
Note that both C
BOOST
and L
BOOST
are available in
surface mount packages.
Table (a) :
Recommended values for C
BOOST
and
L
BOOST
O
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b
V
CC
+
V
2.7
5
10
12
14
18
ro
P
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C
BOOST
L
BOOST
nF
µH
47
100
100
100
220
220
220
220
68
220
470
470
680
1000
I
pk
mA
60
35
33
39
34
31
uc
d
V
rip
mV
190
100
120
220
100
150
150
200
s)
t(
-O
In this case, the external components consist in
one boost inductor, one sense resistor, three ca-
pacitors, and one power MOSFET.
2.5. Typical Telecom Line Cards Protection
Application
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du
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10
9
REF2
I
CC
mA
5
2.5
2.2
2.2
2.4
2.7
C
by
µF
>1
1
1
1
1
1
One of the typical applications where the
TD230
can display all its technical advantages is in an ex-
change Telecom Cards protection. Sometimes fif-
ty cards or more are to be supplied with the same
power supply (+/-5V, 1kW), and a decentralized
protection is needed because one card may be
faulty, but should not penalize the others with un-
adapted protection system. The risk of complete
breakdown of the system must be eradicated.
In this application the two above described over
current causes (external line perturbation or inter-
nal component fault) are likely to happen. In the
first case, the current limitation on each card will
ensure undammaging on-board conditions, and in
the second case, the faulty card will be disjoncted
from the power supply until reset.
Figure 8
shows a typical telecom application with
decentralized protection.
In this application, the positive power supply
serves the logic control and analog signals where-
as the negative power supply is dedicated to the
analog.
4/8
AN1280 - APPLICATION NOTE
Figure 8 :
Decentralized Protection
Vcc+
Power Supply
TD230
TD230
TD230
TD230
GND
BOARD1
BOARD2
BOARD3
BOARDN
Vcc-
O
Therefore, when a fault appears on the positive
rail, the definitive shutdown of the positive NMOS
will lead to the shutdown of the negative NMOS,
but when a fault appears on the negative rail, the
definitive shutdown of the negative NMOS will
have no effect on the positive NMOS.
Several possibilities are offered to reset the whole
system when it has been led to definitive shut-
down :
u
the card can be unplugged and plugged
back (live insertion)
u
the INHIBIT pin can be set to active state
during a short time (100µs typ or more) in
the case of remote control facilities
3. PERFORMANCES AND EVALUATION
All the curves shown in this application note have
been realized with the TD230 Evaluation Board.
The external conditions and components were as
listed hereafter :
u
V
CC
+
= 5V
u
V
CC
-
= -5V
u
Suppliable output short circuit current = 5A
u
IC = TD230
u
MOSFET 1 = BUZ71
u
MOSFET 2 = BUZ71
u
L
BOOST
= 220µH
u
C
BOOST
= 100nF
u
C
TRIP1
= 10µF
u
C
TRIP2
= 10µF
u
R
S1
= 68mΩ
u
R
S2
= 68mΩ
u
C
SS1
= 1nF
u
C
SS2
= 1nF
u
Positive Bypass = 4.7µF (plastic)
u
Negative Bypass = 4.7µF (plastic)
The evaluation board is available and allows to
test the performances of the
TD230.
The layout
and schematic of this evaluation board are given
on
figures 9A-9B-9C.
4. CAUTIONS
For proper use of the
TD230
as a reliable protec-
tion device, a few precautions must be taken :
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1. Proper bypass capacitors must be connected
as close as possible to the power pins of the
TD230
(PV
CC
, NV
CC
, GND). Some recommended
values are given in table (a).
2. The OSCGND pin must be tied to the GND pin
externally (printed board) to ensure proper
step-up converter reference. If not, the step-up
converter will not start.
3. The INHIBIT pin is a CMOS/TTL compatible in-
put which should therefore not be left unconnect-
ed. The absolute maximum rating of this input is
7V. It should be tied to the TTL compatible output
of an eventual control block, or, if it should not be
used, tied to the GND pin.
5/8
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