Datasheet

OP184/OP284/OP484
Rev. J | Page 19 of 24
LOW DROPOUT REGULATOR WITH CURRENT
LIMITING
Many circuits require stable, regulated voltages relatively close
in potential to an unregulated input source. This low dropout
type of regulator is readily implemented with a rail-to-rail output
op amp, such as the OP284, because the wide output swing allows
easy drive to a low saturation voltage pass device. Furthermore,
it is particularly useful when the op amp also employs a rail-to-
rail input feature because this factor allows it to perform high-
side current sensing for positive rail current limiting. Typical
examples are voltages developed from 3 V to 9 V range system
sources or anywhere that low dropout performance is required
for power efficiency. This 4.5 V example works from 5 V nominal
sources with worst-case levels down to 4.6 V or less. Figure 58
shows such a regulator set up, using an OP284 plus a low R
DS(ON)
,
P-channel MOSFET pass device. Part of the low dropout perform-
ance of this circuit is provided by Q1, which has a rating of 0.11
Ω with a gate drive voltage of only 2.7 V. This relatively low gate
drive threshold allows operation of the regulator on supplies as
low as 3 V without compromising overall performance.
The main voltage control loop operation of the circuit is
provided by U1B, half of the OP284. This voltage control
amplifier amplifies the 2.5 V reference voltage produced by
Three Terminal U2, a REF192. The regulated output voltage,
V
OUT
, is then






+=
3
1
R
R2
VV
2
OUTOUT
For this example, because V
OUT
of 4.5 V with V
OUT2
= 2.5 V requires
a U1B gain of 1.8 times, R3 and R2 are chosen for a ratio of 1.2:1 or
10.0 kΩ:8.06 kΩ (using closest 1% values). Note that for the lowest
V
OUT
dc error, R2||R3 should be maintained equal to R1 (as in
this example), and the R2 to R3 resistors should be stable, close
tolerance metal film types. The table in Figure 58 summarizes
R1 to R3 values for some popular voltages. However, note that,
in general, the output can be anywhere between V
OUT2
and the
12 V maximum rating of Q1.
While the low voltage saturation characteristic of Q1 is a key part
of the low dropout, another component is a low current sense com-
parison threshold with good dc accuracy. Here, this is provided by
Current Sense Amplifier U1A, which is provided by a 20 mV
reference from the 1.235 V, AD589 Reference Diode D2, and the
R7 to R8 divider. When the product of the output current and the
R
S
value match this voltage threshold, the current control loop is
activated, and U1A drives the Q1 gate through D1. This causes the
overall circuit operation to enter current mode control with a
current limit, I
LIMIT
, defined as
( )






+








=
R8R
R7
R
V
I
S
D2R
LIMIT
7
3
2
1
8
4
U1A
OP284
U1B
OP284
D2
AD589
D1
1N4148
Q1
SI9433DY
6
5
7
D3
1N4148
2
6
4
3
U2
REF192
R3
10kΩ
V
C
V
IN
COMMON
+V
S
V
S
> V
OUT
+ 0.1V
C4
0.1µF
C5
0.01µF
C2
1µF
C1
0.01µF
C6
10µF
V
OUT
COMMON
V
OUT
=
4.5V @ 350mA
(SEE TABLE)
C2
0.1µF
OPTIONAL
ON/OFF CONTROL INPUT
CMOS HI (OR OPEN) = ON
LO = OFF
V
OUT
R1kΩ R2kΩ R3kΩ
OUTPUT TABLE
5.0V 4.99 10.0 10.0
4.5V 4.53 8.08 10.0
3.3V 2.43 3.24 10.0
3.0V 1.69 2.00 10.0
R5
22.1kΩ
R4
2.21kΩ
R6
4.99kΩ
R9
27.4kΩ
R11
1kΩ
R10
1kΩ
R1
4.53kΩ
V
OUT2
2.5V
R7
4.99kΩ
R8
301kΩ
R2
8.06kΩ
R
S
0.05Ω
00293-058
Figure 58. Low Dropout Regulator with Current Limiting\