OP484FS datasheet(19/24 Pages) AD

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OP184/OP284/OP484

Rev. D | Page 19 of 24

LOW DROPOUT REGULATOR WITH CURRENT

LIMITING

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 V

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.

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

and the 12 V maximum rating of Q1.

OUT2

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 R

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.

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

()

⎟

⎠

⎞

⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

LIMIT

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 V

is then

OUT

⎟

⎠

⎞

⎜

⎝

⎛ +

OUT

U1A

OP284

U1B

OP284

AD589

1N4148

SI9433DY

1N4148

REF192

10kΩ

VIN COMMON

+VS

VS > VOUT + 0.1V

0.1µF

0.01µF

1µF

0.01µF

10µF

VOUT COMMON

VOUT =

4.5V @ 350mA

(SEE TABLE)

0.1µF

OPTIONAL

ON/OFF CONTROL INPUT

CMOS HI (OR OPEN) = ON

LO = OFF

VOUT

R1kΩ

R2kΩ

R3kΩ

OUTPUT TABLE

5.0V

4.99

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

22.1kΩ

2.21kΩ

4.99kΩ

27.4kΩ

R11

1kΩ

R10

1kΩ

4.53kΩ

VOUT2

2.5V

4.99kΩ

301kΩ

8.06kΩ

0.05Ω

Figure 58. Low Dropout Regulator with Current Limiting

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