OP184 Datasheet, PDF (15/20 Pages) Analog Devices – Precision Rail-to-Rail Input & Output Operational Amplifiers

OP184/OP284/OP484
µs
100
90
1nF LOAD
ONLY
SNUBBER
IN 10
CIRCUIT 0%
50mv 50mv
2µs
Figure 54. Overshoot and Ringing Is Reduced by Adding a
“Snubber” Network in Parallel with the 1 nF Load
Table I. Snubber Networks for Large Capacitive Loads
Load Capacitance
(C_L)
1 nF
10 nF
100 nF
Snubber Network
(R_S, C_S)
50 Ω, 100 nF
20 Ω, 1 µF
5 Ω, 10 µF
A 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 out-
put op amp such as the OP284, because the wide output swing
allows easy drive to a low saturation voltage pass device. Fur-
thermore, it is particularly useful when the op amp also enjoys a
rail-rail input feature, as this factor allows it to perform high-
side current sensing for positive rail current limiting. Typical ex-
amples are voltages developed from 3 V to 9 V range system
sources, or anywhere where low dropout performance is required
for power efficiency. The 4.5 V case here works from 5 V nomi-
nal sources, with worst-case levels down to 4.6 V or less.
Figure 55 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 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 compromise to over-
all performance.
The circuit’s main voltage control loop operation is provided by
U1B, half of the OP284. This voltage control amplifier ampli-
fies the 2.5 V reference voltage produced by three terminal U2,
a REF192. The regulated output voltage V_OUT is then:
( ) ^VOUT ^=VOUT 2
1+
R2
R3
For the example here, a V_OUT of 4.5 V with V_OUT2 = 2.5 V re-
quires a U1B gain of 1.8 times, so 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, R2ʈR3 should be main-
tained equal to R1 (as here), and the R2-R3 resistors should be
stable, close tolerance metal film types. The table in Figure 55
summarizes R1-R3 values for some popular voltages. However,
note that in general the output can be anywhere between V_OUT2
to 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 a
20 mV reference from the 1.235 V AD589 reference diode D2
and the R7-R8 divider. When the product of the output current
and the R_S value matches this voltage threshold, the current
control loop is activated, and U1A drives Q1’s gate through D1.
This causes the overall circuit operation to enter current mode
control, with a current limit I_LIMIT defined as:
( ) ^ILIMIT
=

V_R(D2)
R_S

R7
R7 + R8
+V_S
V_S > V_OUT + 0.1V
C3
0.1µF
V_C
OPTIONAL
ON/OFF CONTROL INPUT
CMOS HI (OR OPEN) = ON
LO = OFF
V_IN COMMON
D2
AD589
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
R2
OP284
8.06kΩ
U2
2 ^REF192
6
3
R10
C2
4
1kΩ
1µF
V_OUT2
2.5V
R3
10kΩ
OUTPUT TABLE
V_OUT R1 R2 R3
5.0V 4.99k 10.0k 10.0k
4.5V 4.53k 8.06k 10.0k
3.3V 2.43k 3.24k 10.0k
3.0V 1.69k 2.00k 10.0k
V_OUT =
4.5V @ 350mA
(SEE TABLE)
C6
10µF
V_OUT COMMON
REV. 0
Figure 55. A Low Dropout Regulator with Current Limiting
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