MC3423
Overvoltage Crowbar
Sensing Circuit
This overvoltage protection circuit (OVP) protects sensitive
electronic circuitry from overvoltage transients or regulator failures
when used in conjunction with an external “crowbar” SCR. The
device senses the overvoltage condition and quickly “crowbars” or
short circuits the supply, forcing the supply into current limiting or
opening the fuse or circuit breaker.
The protection voltage threshold is adjustable and the MC3423 can
be programmed for minimum duration of overvoltage condition
before tripping, thus supplying noise immunity.
The MC3423 is essentially a “two terminal” system, therefore it
can be used with either positive or negative supplies.
Features
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MARKING
DIAGRAMS
8
1
PDIP−8
P1 SUFFIX
PLASTIC PACKAGE
CASE 626
MC3423P1
AWL
YYWW
•
Pb−Free Package is Available
MC3423P1 = Device Code
A
= Assembly Location
WL
= Wafer Lot
YY
= Year
WW
= Work Week
MAXIMUM RATINGS
Rating
Differential Power Supply Voltage
Sense Voltage (1)
Sense Voltage (2)
Remote Activation Input Voltage
Output Current
Operating Ambient Temperature Range
Operating Junction Temperature
Storage Temperature Range
Symbol
V
CC
−V
EE
V
Sense1
V
Sense2
V
act
I
O
T
A
T
J
T
stg
Value
40
6.5
6.5
7.0
300
0 to +70
125
−65 to +150
Unit
Vdc
Vdc
Vdc
Vdc
mA
°C
°C
°C
8
1
8
SOIC−8
D SUFFIX
PLASTIC PACKAGE
CASE 751
3423
ALYW
1
Maximum ratings are those values beyond which device damage can occur.
Maximum ratings applied to the device are individual stress limit values (not
normal operating conditions) and are not valid simultaneously. If these limits
are exceeded, device functional operation is not implied, damage may occur
and reliability may be affected.
3423
A
L
Y
W
= Device Code
= Assembly Location
= Wafer Lot
= Year
= Work Week
PIN CONNECTIONS
Drive
Output
V
EE
Indicator
Output
Remote
Activation
V
in
Current
Limited
DC
Power
Supply
+
C
out
V
out
V
CC
1
Sense 1 2
8
7
6
5
(Top View)
O. V. P.
MC3423
Sense 2 3
Current
4
Source
Figure 1. Simplified Application
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 7 of this data sheet.
©
Semiconductor Components Industries, LLC, 2005
1
March, 2005 − Rev. 5
Publication Order Number:
MC3423/D
MC3423
ELECTRICAL CHARACTERISTICS
(5.0 V
≤
V
CC
− V
EE
≤
36 V, T
low
< T
A
, T
high
, unless otherwise noted.)
Characteristics
Supply Voltage Range
Output Voltage (I
O
= 100 mA)
Indicator Output Voltage (I
O(Ind)
= 1.6 mA)
Sense Trip Voltage (T
A
= 25°C)
Temperature Coefficient of V
Sense1
(Figure 2)
Remote Activation Input Current
(V
IH
= 2.0 V, V
CC
− V
EE
= 5.0 V)
(V
IL
= 0.8 V, V
CC
− V
EE
= 5.0 V)
Source Current
Output Current Risetime (T
A
= 25°C)
Propagation Delay Time (T
A
= 25°C)
Supply Current
NOTES: T
low
to T
high
= 0° to +70°C
V
CC
1
Symbol
V
CC
−V
EE
V
O
V
OL
(Ind)
V
Sense1,
V
Sense2
TCV
S1
I
IH
I
IL
I
Source
t
r
t
pd
I
D
Min
4.5
V
CC
−2.2
−
2.45
−
−
−
0.1
−
−
−
Typ
−
V
CC
−1.8
0.1
2.6
0.06
5.0
−120
0.2
400
0.5
6.0
Max
40
−
0.4
2.75
−
40
−180
0.3
−
−
10
mA
mA/ms
ms
mA
Unit
Vdc
Vdc
Vdc
Vdc
%/°C
mA
I
Source
2
Sense 1
4 Current
Source
+
−
8
Output
−
+
V
ref
2.6V
−
+
7
V
EE
3
Sense 2
5
6
Remote
Activation
Indicator
Output
Figure 2. Representative Block Diagram
V
CC
Switch 1
(A)
(B)
Switch 2
V
I
1
2
3
4
7
5
8
Switch 1
V
Sense 1
V
Sense 2
V
Ramp V
I
until output goes high; this is
the V
Sense
threshold.
Position A
Position B
Switch 2
Closed
Open
MC3423
Figure 3. Sense Voltage Test Circuit
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2
MC3423
*
F1
R1
Power
Supply
R2
2
3
4
(+ Sense
Lead)
1
MC3423
7 5
S1*
(− Sense Lead)
−
*See text for explanation.
8
R
G
To
Load
+
Vtrip
+
Vref 1
)
R1
R2
[
2.6 V 1
)
R1
R2
R2
≤
10 kW for minimum drift
For minimum value of R
G
, see Figure 9.
Figure 4. Basic Circuit Configuration
+
R
S
1
8
Power
Supply
1N4740
10V
+
10mF
15V
MC3423
2
3
4
7
5
*R2
(− Sense
Lead)
−
C
1
V
S
(+ Sense
Lead)
R1
Q1
To
Load
C
1
>
RS
+
R
S
(R
1
+ R
2
) 10mF
R
1
R
2
VS – 10
kW
25
[
2.6 V 1
)
R
R
Vtrip
+
Vref 1
)
R1
R2
*R2
≤
10 kW
Q1:
Q1:
Q1:
Q1:
Q1:
Q1:
V
S
≤
50 V; 2N6504 or equivalent
V
S
≤
100 V; 2N6505 or equivalent
V
S
≤
200 V; 2N6506 or equivalent
V
S
≤
400 V; 2N6507 or equivalent
V
S
≤
600 V; 2N6508 or equivalent
V
S
≤
800 V; 2N6509 or equivalent
Figure 5. Circuit Configuration for Supply Voltage Above 36 V
V
CC
V
trip
+V
CC
R3
R1
Power
Supply
2
1
6
V
10
Indication
8 Out
R
G
V
ref
V
C
0
MC3423
4
3
C
0
V
O
5 7
V
O
0
V
IO
R2
V
C
t
d
R3
≥
V
trip
10 mA
t
d
=
V
ref
I
source
×
C
≈
[12
×
10
3
] C
(See Figure 10)
Figure 6. Basic Configuration for Programmable Duration
of Overvoltage Condition Before Trip
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3
MC3423
APPLICATION INFORMATION
Basic Circuit Configuration
The basic circuit configuration of the MC3423 OVP is
shown in Figure 3 for supply voltages from 4.5 V to 36 V,
and in Figure 4 for trip voltages above 36 V. The threshold
or trip voltage at which the MC3423 will trigger and supply
gate drive to the crowbar SCR, Q1, is determined by the
selection of R1 and R2. Their values can be determined by
the equation given in Figures 3 and 4, or by the graph shown
in Figure 8. The minimum value of the gate current limiting
resistor, R
G
, is given in Figure 9. Using this value of R
G
, the
SCR, Q1, will receive the greatest gate current possible
without damaging the MC3423. If lower output currents
are required, R
G
can be increased in value. The switch, S1,
shown in Figure 3 may be used to reset the crowbar.
Otherwise, the power supply, across which the SCR is
connected, must be shut down to reset the crowbar. If a non
current−limited supply is used, a fuse or circuit breaker, F1,
should be used to protect the SCR and/or the load.
The circuit configurations shown in Figures 3 and 4 will
have a typical propagating delay of 1.0
ms.
If faster
operation is desired, Pin 3 may be connected to Pin 2 with
Pin 4 left floating. This will result in decreasing the
propagating delay to approximately 0.5
ms
at the expense
of a slightly increased TC for the trip voltage value.
Configuration for Programmable Minimum Duration
of Overvoltage Condition Before Tripping
(+ Sense
Lead)
1
R1
Z1
Power
Supply
R2
2
MC3423
5
4 3
1k
C
(− Sense Lead)
7
8 R
G
+
−
Figure 7. Configuration for Programmable
Duration of Overvoltage Condition Before
Trip/With Immediate Trip at
High Overvoltages
Additional Features
In many instances, the MC3423 OVP will be used in a
noise environment. To prevent false tripping of the OVP
circuit by noise which would not normally harm the load,
MC3423 has a programmable delay feature. To implement
this feature, the circuit configuration of Figure 5 is used. In
this configuration, a capacitor is connected from Pin 3 to
V
EE
. The value of this capacitor determines the minimum
duration of the overvoltage condition which is necessary to
trip the OVP. The value of C can be found from Figure 10.
The circuit operates in the following manner: When V
CC
rises above the trip point set by R1 and R2, an internal
current source (Pin 4) begins charging the capacitor, C,
connected to Pin 3. If the overvoltage condition disappears
before this occurs, the capacitor is discharged at a rate
≅
10
times faster than the charging rate, resetting the timing
feature until the next overvoltage condition occurs.
Occasionally, it is desired that immediate crowbarring of
the supply occur when a high overvoltage condition occurs,
while retaining the false tripping immunity of Figure 5. In
this case, the circuit of Figure 6 can be used. The circuit will
operate as previously described for small overvoltages, but
will immediately trip if the power supply voltage exceeds
V
Z1
+ 1.4 V.
1. Activation Indication Output
An additional output for use as an indicator of OVP
activation is provided by the MC3423. This output is an
open collector transistor which saturates when the OVP
is activated. In addition, it can be used to clock an edge
triggered flip−flop whose output inhibits or shuts down
the power supply when the OVP trips. This reduces or
eliminates the heatsinking requirements for the crowbar
SCR.
2. Remote Activation Input
Another feature of the MC3423 is its remote
activation input, Pin 5. If the voltage on this CMOS/TTL
compatible input is held below 0.8 V, the MC3423
operates normally. However, if it is raised to a voltage
above 2.0 V, the OVP output is activated independent of
whether or not an overvoltage condition is present. It
should be noted that Pin 5 has an internal pullup current
source. This feature can be used to accomplish an
orderly and sequenced shutdown of system power
supplies during a system fault condition. In addition, the
activation indication output of one MC3423 can be used
to activate another MC3423 if a single transistor inverter
is used to interface the former’s indication output to the
latter ’s remote activation input, as shown in Figure 7. In
this circuit, the indication output (Pin 6) of the MC3423
on power supply 1 is used to activate the MC3423
associated with power supply 2. Q1 is any small PNP
with adequate voltage rating.
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MC3423
+
1
R1, RESISTANCE (k
Ω
)
Power
Supply
#1
7
6
20
30
Typ
R2 = 2.7 k
Max
Min
−
R1
10k
+
Q1
5
1.0k
10
1
Power
Supply
#2
7
0
0
5.0
10
15
20
V
T
, TRIP VOLTAGE (V)
25
30
−
Figure 9. R1 versus Trip Voltage
Figure 8. Circuit Configuration for
Activating One MC3423 from Another
35
VCC , SUPPLY VOLTAGE (V)
Note that both supplies have their negative output
leads tied together (i.e., both are positive supplies). If
their positive leads are common (two negative supplies)
the emitter of Q1 would be moved to the positive lead of
supply 1 and R1 would therefore have to be resized to
deliver the appropriate drive to Q1.
Crowbar SCR Considerations
30
25
20
15
10
R
G(min)
= 0
if V
CC
< 11 V
C, CAPACITANCE (
µ
F)
Referring to Figure 11, it can be seen that the crowbar
SCR, when activated, is subject to a large current surge
from the output capacitance, C
out
. This capacitance
consists of the power supply output caps, the load’s
decoupling caps, and in the case of Figure 11A, the supply’s
input filter caps. This surge current is illustrated in Figure
12, and can cause SCR failure or degradation by any one
of three mechanisms: di/dt, absolute peak surge, or I
2
t. The
interrelationship of these failure methods and the breadth
of the applications make specification of the SCR by the
semiconductor manufacturer difficult and expensive.
Therefore, the designer must empirically determine the
SCR and circuit elements which result in reliable and
effective OVP operation. However, an understanding of the
factors which influence the SCR’s di/dt and surge
capabilities simplifies this task.
di/dt
0
10
20
30
40
50
60
70
RG, GATE CURRENT LIMITING RESISTOR (W)
80
Figure 10. Minimum R
G
versus Supply Voltage
1.0
1
2 3 5 7 1
0.1
0.01
0.001
As the gate region of the SCR is driven on, its area of
conduction takes a finite amount of time to grow, starting
as a very small region and gradually spreading. Since the
anode current flows through this turned−on gate region,
very high current densities can occur in the gate region if
high anode currents appear quickly (di/dt). This can result
in immediate destruction of the SCR or gradual
degradation of its forward blocking voltage capabilities −
depending on the severity of the occasion.
0.0001
0.001
1
5
2
1
0.01
0.1
t
d
, DELAY TIME (ms)
1.0
10
Figure 11. Capacitance versus
Minimum Overvoltage Duration
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