LINEAR TECHNOLOGY LT6108-2 Technical data
Simple Energy-Tripped Circuit Breaker with Automatic
Delayed Retry
Design Note 495
Tim Regan
Introduction
A circuit breaker protects sensitive load circuits from
excessive current flow by opening the power supply when
the current reaches a predetermined level. The simplest
circuit breaker is a fuse, but blown fuses require physical
replacement. An electronic circuit breaker provides the
same measure of circuit protection as a fuse without the
single-use problem. Nevertheless, an electronic circuit
breaker with a fixed trip current threshold, while effective
for protection, can become a nuisance if tripped by short
duration current transients—even if the circuit breaker
self-resets.
One way to minimize nuisance breaks is to employ a
slow-blow technique, which allows relatively high levels
of current for short intervals of time without tripping the
breaker. Ideally, the breaker’s trip threshold would be a
function of total transient energy, instead of just current.
This article describes an electronic circuit breaker, combining current sensing with timing to create an energytripped breaker, which protects sensitive circuits while
minimizing nuisance trips.
LOAD SUPPLY
5V TO 80V DC
R
SENSE
10mΩ
CIRCUIT BREAKER
Q1
NPN
LOAD
0.667V/A
R1
100Ω
LT6108-2
CURRENT
SENSE AMP
–
+
COMPARATOR
5V
L
RELAY
R14
15k
400mV
CLOSED
5
0
–
+
OPEN
R4
40.2k
R2
1.62k
R3
6.34k
CURRENT SENSE
COMPARATOR
500mA TRIP
THRESHOLD
Higher Currents Permitted for Shorter Time Intervals
The circuit of Figure 1 has three distinct parts – circuit
breaking, current sensing and timing.
The circuit breaking function can be any t ype of electronically controlled relay or solid state switch, properly sized
for voltage and current rat ings of the load being protecte d.
Load current sensin g is achieved via an LT®6108-2 current
sense amplifier with built-in comparator. The LT6108-2
converts the voltage drop across a small valued sense
resistor to a ground-referenced output voltage that is
directly pr oportional to the load curr ent. The trip threshold
is created by s caling the output voltage via resis tor divider
and feeding the result to the integrated comparator with
a precision 400mV voltage reference. The comparator
changes st ate when the load current exce eds the threshold.
To prevent short duration transients from causing nui-
®
sance trips, an LTC
L, LT, LTC, LTM, Linear Technology, the Linear logo and Timerblox are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
R3
3.66k
5V
–
LT1783
+
5V
R10
10k
6994-2 Timerblox® delay timer is
60ms TO 350ms
DELAY TIMER
R6
64.9k
R7
301k
IN
GND
SET
LTC6994-2
R
OPTIONAL
931k
OUT
DIV
+
V
R8
172k
R9
100k
5V
DN495 F01
10/11/495
Figure 1. Energy-Tripped Circuit Breaker Trips After a Time Interval
That Varies as a Function of Sensed Load Current
added between the comparator output and the circuit
breaker. Once tripped, the comp arator falling edge star ts a
variable time delay interval, which, if allowed to complete,
signals the circuit breaker to open. Nothing happens if
the transient duration is shorter than the delay.
A Current-Controlled Delay Interval
The LTC6994-2 delays from an edge appearing at its IN
pin by a time ranging from 1µs to 33 sec. The delay time
is controlled by the current sourced by the SET pin, which
programs an internal oscillator frequency, while the bias
voltage on the DIV pin selects a frequency divide ratio.
The LT1783 op amp circuit takes the output voltage
from the current sense amplifier and adjusts the SET pin
current, thereby making the delay time a function of the
load current (see Figure 2). As shown, the current sense
comparator trip threshold is 500mA. A current of 500mA
creates a falling edge and starts a time delay of 350ms.
Should the load current drop below 500mA before the
delay time expires, the timer output remains high and the
circuit breaker does not trip.
Higher load currents correspond to higher current sense
amplifier output voltages, which in turn reduce the delay
time interval (Figure 2). For instance, a 5A load current
trips the circuit breaker in only 60ms. Depending on the
average load current in excess of the 500mA threshold,
the delay inter val or trip time will fall somewhere between
30ms and 400ms.
output voltage stretches this delay to a maximum time
of ~1.3 sec. After this delay the circuit breaker closes and
reapplies power to the load. This automatic retry function
requires no additional components.
The response of the circuit to a 5A load current spike and
automatic retry is shown in Figure 3. If the load current
remains too high, the trip/r etry cycle repea ts continually. A
current surge is fairly common when the circuit breaker is
first closed and can trip the comparator. If the duration is
less than the timer delay, the breaker remains closed, thus
avoiding an endless loop of self-induced nuisance trips.
A B
1V
5V
5V
Figure 3. An Example Trip and Retry Sequence. At Time
Point A, the 5A Load Current Spike Trips the Comparator
and 60ms Later the Breaker is Opened. At Time B, After
a Delay Time of 1.3 sec, the Timer Closes the Breaker.
The Resulting Short Duration Spike of Start-Up Current Is
Not Large Enough or Long Enough in Duration to Trip the
Breaker Again
CURRENT SENSE OUTPUT
CURRENT COMPARATOR OUTPUT
DELAY TIMER OUTPUT
200ms/DIV
DN495 F03
Once tripped, the load current drops to zero. This resets
the current sens e comparator high. This rising edg e is also
delayed by the LTC6994-2. The minimum current sense
400
350
300
250
200
150
100
BREAKER DELAY TIME (ms)
50
0
0
31 98
LOAD CURRENT (A)
Figure 2. Low Current Transients Must Last Relatively
Longer to Trip the Breaker. Higher Currents Trip the Circuit
Breaker in Less Time
Data Sheet Download
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11102 4 5 6 7
DN495 F02
Extending the Retry Time Interval
The LTC6994-2 delay timer has eight divider settings for a
wide range of timing intervals. Adding the single optional
resistor shown in Figure 1 shifts the delay block to a new
setting, increasing the retry time interval if desired. This
can give any fault condition more time to subside. The
circuit breaker response time interval is not affected.
For the values shown, when the circuit breaker trips and
the current drops to zero, the c omparator high level biases
the DIV pin to a higher voltage level, resulting in a longer
retry delay time of 10 seconds.
Conclusion
The circuit shown here can be easily modified to different
timing requirements with a few resistor value changes.
Other current sense devices such as the LT1999 can
also be used to monitor bidirectional load currents with
variable breaker timing functionality.
For applications help,
call (408) 432-1900, Ext. 3409
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
dn495f LT/AP 1011 196K • PRINTED IN THE USA
LINEAR TECHNOLOGY CORPORATION 2011
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