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 RDS(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,

VOUT, is then











 +

=

3

1

R

R2

V

V

2

OUT

OUT

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 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

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

( )













+













=

R8

R

R7

R

V

I

S

D2

R

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

10k

Ω

VC

VIN COMMON

+VS

VS > VOUT + 0.1V

C4

0.1µF

C5

0.01µF

C2

1µF

C1

0.01µF

C6

10µF

VOUT COMMON

VOUT =

4.5V @ 350mA

(SEE TABLE)

C2

0.1µF

OPTIONAL

ON/OFF CONTROL INPUT

CMOS HI (OR OPEN) = ON

LO = OFF

VOUT

R1k

Ω 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

22.1k

Ω

R4

2.21k

Ω

R6

4.99k

Ω

R9

27.4k

Ω

R11

1k

Ω

R10

1k

Ω

R1

4.53k

Ω

VOUT2

2.5V

R7

4.99k

Ω

R8

301k

Ω

R2

8.06k

Ω

RS

0.05

Ω

Figure 58. Low Dropout Regulator with Current Limiting\