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Feb 2017
AN53U Rev A
External Current Limit for Apex Power Op Amps
AN53
INTRODUCTION
Apex AN09, “Current Limiting”, discusses the basic internal current limit function of Apex operational
amplifiers. Although most Apex op amps have a built-in current limit function, some devices (like PA107,
PA74/PA76 or PA75) will require external circuitry when current limit function is needed. This article will dis-
cuss some possible solutions with simulation and lab test results.
EXTERNAL CURRENT LIMIT CIRCUIT LOCATIONS
With the objective of delivering desired power to the load and reducing the risk of violating the op amp’s
SOA, the external current limit circuit can be placed either at the output of an application, or at the power
supply rails of the op amp.
When placed at the output of an application, a current limit circuit can be located either outside of the op
amp’s closed-loop (Figure 1a), or inside of the closed-loop (Figure 1b). The maximum current limited by the
current limit circuit will be the same for both configurations, with slightly different behaviors.
The first connection allows the op amp circuit to be designed as normal without a current limit circuit and
then add the current limit circuit. The final system will have the same original performance of the op amp cir-
cuit (in term of frequency response, phase shift, etc.), with a slightly lower output voltage delivered to the
load which is caused by the voltage drop of the current limit circuit. This configuration is also good for adding
a current limit circuit to an existing system. It can be designed as a 2-terminal component and inserted
between the op amp’s output and the load without re-spinning the main system board.
If the load requires a precise voltage, and the voltage drop caused by the current limit circuit is not
acceptable for the application, the configuration in Figure 1b can be used. With the current limit circuit
included in the closed-loop, the voltage drop across the current limit circuit will be compensated by the sys-
tem, but the characteristics of the circuit could be different from the original circuit. Therefore, the stability
and other performance of the new system should be evaluated.
When placed at the power supply rails (Figure 2), the current limit circuits will limit the current provided
to the op amp which also means lower the supply voltages to the op amp.
Figure 1: Current Limit Circuit at Output
A.) Current limit circuit located outside the closed-
loop
B.) Current limit circuit located inside the closed-
loop
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Circuit
+V
S
+IN
-IN
-V
S
OUT
R
f
LOAD
-V
+V
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Circuit
+V
S
+IN
-IN
-V
S
OUT
R
f
LOAD
-V
+V
AN53
2 AN53U Rev A
Figure 2: Current Limit Circuit in the Supply Rails
EXTERNAL CURRENT LIMIT CIRCUIT TOPOLOGIES
BJT CIRCUIT FOR EXTERNAL CURRENT LIMITING
In 1991 Apex introduced a BJT based circuit for external current limit function, as shown in Figure 3. Two
circuits are used as current limit circuit and are inserted between the op amp’s power supply pins and the
supply rails. The circuits operate in the same way as built-in current limit circuitry, as discussed in AN09.
N-CHANNEL DEPLETION MODE MOSFETS FOR EXTERNAL CURRENT LIMIT CIRCUIT
The BJT transistors in Figure 3 can be replaced by N-channel Depletion Mode MOSFETs to build a simpler
current limit circuit as shown in Figure 4. Unlike Enhancement Mode MOSFETs, which are “normally-off”
devices, Depletion Mode MOSFETs are “normally-on” and stop to conduct when V
GS
exceeds a certain value.
Figure 3: Typical BJT Current Limit Circuit
A.) BJT positive rail current limit circuit
B.) BJT negative rail current limit circuit
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Circuit
+V
S
+IN
-IN
-V
S
OUT
LOAD
-V +V
ƵƌƌĞŶƚ>ŝŵŝƟŶŐ
Circuit
R1
Rcl1
Q1
Q2
R2
C1
+V
To_op-amp_+Vs
I+
R4
Rcl2
Q3
Q4
R3
C2
-V
To_op-amp_-Vs
I-