廬
APPLICATION BULLETIN
AN EASY SOLUTION TO CURRENT
LIMITING AN OP AMP
By Richard Kulavik
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (520) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Immediate Product Info: (800) 548-6132
1
Many circuits today not only require voltage regulation but
they also need current regulation. Some systems draw ex-
cessive current during power on, which can cause expensive
system components to burn up due to improper powering of
supplies. This is not the only problem associated with
excessive currents. Some applications need to limit the
current an op amp can provide to a specific load. This circuit
can provide this type of function by making constant current
sources out of simple regulators.
The REG1117 is a low cost, low drop out, adjustable
regulator that can be used for both voltage and current
regulation. In the normal mode of operation, applying a
fixed voltage on the input pin will provide a fixed voltage
between V
OUT
and Adj. Placing a resistor across V
OUT
and
Adj will limit the amount of current delivered to the load by
making the regulator appear as a constant current source.
+V
S
3
U1
REG1117
1 Adj
50碌A(chǔ)
2 V
OUT
I
REG
R
1
Rf
Figure 1 shows two REG1117s (U
1
, U
2
) with R
1
, from the
V
OUT
pins to the Adj pins allowing
I
REG(MAX)
= (1.25V/R
1
) 鈥?50碌A(chǔ)
to be sources from the floating output to the system. If the
system does not sink or source more than or equal to I
REG(MAX)
,
the regulators will be out of regulation and will drop its
voltage across V
OUT
, to Adj equalling 1.25V + I
REG
鈥?R
1
,
where I
REG
is equal to the current demands of the system.
When the current demands of the system rise and approach
1.25V/R
1
, the regulator will begin to maintain its regulation
and provide a maximum current, limited by R
1
. At this point,
the maximum voltage drop will be equal to 1.2V + 1.25V or
2.45V from V
IN
to Adj. The supply voltage minus 2.45V will
determine the compliance voltage of the current regulator.
The REG1117 can source from 10mA to 800mA using the
circuits shown.
Two problems can arise from creating a circuit such as the
one described in Figure 1. The first problem is that the
REG1117s have a transient response that will affect the
output of the amplifier. Figure 2 shows the output voltage
deviation in millivolts versus time from the REG1117. This
voltage transient will appear as a ringing voltage to the load
of the amplifier. If bypass capacitors are added on the power
supply pins of an amplifier (A
1
), the ringing can be reduced
at the cost of greater overshoot and longer settling times.
Remember that bypass capacitors are on the supply leads of
3
4
C
BYPASS
LINE TRANSIENT RESPONSE
60
Output Voltage
Deviation (mV)
Load
System
A1
40
20
0
鈥?0
鈥?0
C
IN
= 1碌F
C
OUT
= 10碌F Tantalum
I
OUT
= 0.1A
3
C
BYPASS
U2
REG1117
Adj 1
V
OUT
2
R
1
I
REG
Input
Voltage (V)
5.25
4.25
3.25
0
20
40
60
80
100 120
Time (碌s)
140 160
180 200
鈥揤
S
FIGURE 1. A Voltage Regulator Can Be Used as a Current
Regulator and an Op Amp.
FIGURE 2. Transient Response of the REG1117.
漏
1995 Burr-Brown Corporation
AB-099
Printed in U.S.A. May, 1995
1
2
