DN298 - The LT1970 Op Amp Provides On-The-Fly Adjustable Current Limit for Flexibility and Load Protection in High Current Applications

Power Op Amp Provides On-the-Fly
Adjustable Current Limit for Flexibility and Load Protection
in High Current Applications
Design Note 298
Tim Regan
Introduction
Many power operational amplifiers offer a built-in
current limit where the limit is fixed or programmable
through an external resistor. This offers the most basic
measure of protection for the load circuitry, and the
amplifier itself, under fault conditions. Sometimes,
though, there is a need for on-the-fly current limiting to
satisfy the requirements of different loads. For example,
automatic test equipment (ATE) systems use multiple
pin drivers to deliver test voltages across a wide range
of loads, including faults, to a unit or board to test for
continuity or functionality. To protect the load circuitry,
the ATE must precisely control the maximum current
delivered to each pin. Ideally the maximum current can
be controlled on the fly, to accommodate the different
loads at each pin.
output current anywhere from 4mA to 500mA. Even
at 500mA, the accuracy of the output current limit is
guaranteed to a tight 2% (10mA) tolerance. The output
current is continuously sensed by a small valued resistor, RS, connected in series with the load as shown in
Figure 1. The maximum output current is a function
of the control-input voltage and the sense resistor
according to the following expression:
IOUT(LIMIT) =
By simply changing the voltage between 0V and 5V at
the control input, say through a D-to-A converter, the
output current limit quickly changes to a new level.
For example, Figure 2 shows the output waveforms
of the LT1970 driving a 1μF capacitive load in parallel
with a 100Ω resistor. The current limit is set to 500mA
sourcing (VCSRC = 5V) and 50mA sinking (VCSNK =
500mV). To charge the load capacitance, the amplifier
current limits until the output voltage reaches its proper
Introducing the LT1970
The LT®1970 op amp can supply ±500mA of output
current, with a precise, easy-to-implement current
limit. Current limit control is via two simple 0V–5V
voltage inputs, VCSRC (for sourcing current) and
VCSNK (for sinking current), that set the maximum
500mV
CURRENT
SINK
LIMIT
CONTROLS SOURCE
5V
(0V–5V)
5V
–5V
VIN
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
12V
VCSRC
VCSNK
EN
+IN
10k
R6
10k
–IN
VCC
V+
ISNK
ERROR FLAGS
1.5k INDICATE
CURRENT
LIMIT AND
THERMAL
SHUTDOWN
+
0.1μF
10μF
VOUT
5V/DIV
0V
RS
1
ISRC
TSD
OUT
LT1970
SENSE+
SENSE–
V–
VEE
R6
100Ω
+
VOUT
CL
1μF
IOUT
200mA/DIV
0mA
COMMON
300pF
+
0.1μF
–12V
10μF
200μs/DIV
RF
10k
DN298 F01
Figure 1. The LT1970: Easy to Use as an Op Amp
with an Adjustable Output Current Power Stage
10/02/298_conv
VLIMIT
t3S
DN298 F02
Figure 2. 500mA Source Current Limit
and 50mA Sink Current Limit Control
Output Response Characteristics
closed loop value. Then, while swinging negative, the
sinking current limit prevents the output from going
less than –5V.
The LT1970 also features open-collector error flags.
These three outputs indicate that the amplifier is in
current limit, either sourcing or sinking, and that the
amplifier is in thermal shutdown. Additionally, the Enable input can be used to turn off the amplifier, thus
putting the output into a high-impedance, zero output
current state. This same input can also be used to
simultaneously apply a new set of voltage and current
settings to the load. The LT1970 is available in a small
20-pin TSSOP package with exposed underside metal
for heat dissipation.
Boosted Output Current with “Snap-Back”
Current Limiting
The LT1970 has separate supply pins for the input
stage and the power output stage. Only load current
flows through the output stage power supplies (V+
and V–). These pins can provide gate or base drive to
external power transistors to boost the output current
capability of the amplifier. A simple power stage, shown
in Figure 3, increases the output current to ±5A. The
same 0V to 5V inputs now set the output current limits
a factor of ten higher (to 1A/V) by the use of a smaller
current sense resistor, RS = 0.1Ω.
Externally connected gain setting resistors allow Kelvin
sensing at the load. By connecting the feedback resis12V
R1
100k
R2
20k
R5
51Ω
R3
1.4k
VCSRC
VCSNK
ISNK
+IN
ISRC
VIN
R4
10k
EN
SENSE+
–IN
SENSE–
COMMON
VCC
R8
50Ω
V+
RSENSE
0.1Ω FORCE
VOUT =
±10V at
±5A
OUT
VEE
V–
R7
16Ω
VIN
5V/DIV 0V
LOAD
VOUT
5V/DIV 0V
Q1
D44VH10
R6
51Ω
RF
10k
Conclusion
The LT1970 is a versatile and easy to use power op
amp with a built-in precision adjustable current limit,
which can protect load circuitry from damage caused
by excessive power from the amplifier. This feature
is particularly useful in ATE systems where the load
is variable (and possibly faulty) at each tested node.
Tight control of the output current in these systems
is important to prevent damage to the tested unit.
Q1
D45DH10
LT1970
RG
10k
tor right at the load, the voltage placed on the load
is exactly what it should be. Any voltage drop across
the current sense resistor is inside the feedback loop
and thus does not create a voltage error. Figure 3
also shows a unique way to use the open-collector
error flags to provide extra protection to the load
circuitry. When the amplifier enters current limit in
either direction, the appropriate error flag goes low.
This high impedance to 0V transition can provide a
large amount of hysteresis to the current limit control
inputs, forcing a drastic reduction in output current.
Resistors R1, R2 and R3 set the current limit control
feedback at 2V max and 200mV min. Should the load
current ever exceed the predetermined maximum
limit, the output current snaps back to the min level.
The output current remains at this lower level until
the signal drops to a point where the load current is
less than the minimum set value. When the signal is
low enough, the flag output goes open and the current limit reverts to the maximum value. This action
simulates an automatically resettable fuse to protect a
load. Figure 4 shows the action of this feedback with a
maximum current limit of 2A snapping back to 200mA
when exceeded in either direction.
SENSE
–12V
DN298 F03
Figure 3. Easily Adjusted Current Limit
for a ±5A Boosted Output Current Stage
Data Sheet Download
www.linear.com
Linear Technology Corporation
RLOAD = 5Ω
500μs/DIV
DN298 F04
Figure 4. “Snap Back” Current Limiting with
Both Source and Sink Current Limit Controlled
by a Simple Resistor Network
For applications help,
call (408) 432-1900
dn298f_conv LT/TP 1002 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2002
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