OP184_06 Datasheet, PDF (19/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
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 RDS(ON), P-channel MOSFET pass device. Part
of the low dropout performance 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 circuit’s main voltage control loop operation 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 VOUT is then
V_OUT
= V_{OUT2 ⎜⎝⎛}1 +
R2
R3
⎟⎠⎞
For this example, because VOUT of 4.5 V with VOUT2 = 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 VOUT 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 VOUT2 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 comparison 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 RS 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, ILIMIT, defined
as
I _LIMIT
=
⎜⎜⎝⎛
V_R(D
R_S
2
)
⎟⎟⎠⎞⎜⎝⎛
R7
R7 + R8
⎟⎠⎞
+V_S
V_S > V_OUT + 0.1V
D2
AD589
C2
0.1µF
V_C
OPTIONAL
ON/OFF CONTROL INPUT
CMOS HI (OR OPEN) = ON
LO = OFF
V_IN COMMON
R_S
0.05Ω
C4
0.1µF
Q1
SI9433DY
R7
4.99kΩ
R8
301kΩ
R6
4.99kΩ
3
2
U1A
OP284
8
1
4
R9
27.4kΩ
D3
1N4148
C5
0.01µF
R11
1kΩ
R1
4.53kΩ
D1
1N4148
R5
22.1kΩ
C1
0.01µF
R4
2.21kΩ
6
7
5
U1B
OP284
R2
8.06kΩ
U2
2 ^REF192
6
3
R10
C2
4
1kΩ
1µF
V_OUT2
2.5V
V_OUT
R3 5.0V
10kΩ 4.5V
3.3V
3.0V
OUTPUT TABLE
R1kΩ R2kΩ R3kΩ
4.99 10.0 10.0
4.53 8.08 10.0
2.43 3.24 10.0
1.69 2.00 10.0
Figure 58. Low Dropout Regulator with Current Limiting
V_OUT =
4.5V @ 350mA
(SEE TABLE)
C6
10µF
V_OUT COMMON
Rev. D | Page 19 of 24