OP484ES datasheet(15/20 Pages) AD

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REV. 0

–15–

OP184/OP284/OP484

100

1nF LOAD

ONLY

SNUBBER

CIRCUIT

2µs

50mv

µs

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

Snubber Network

(CL)(RS, CS)

1 nF

Ω, 100 nF

10 nF

Ω, 1 µF

100 nF

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 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 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 VOUT is then:

VOUT =VOUT 2 1 +

R 3

()

For this example, since VOUT of 4.5 V with VOUT2 = 2.5 V re-

quires a U1B gain of 1.8 times, R3 and R2 are chosen for a ratio

of 1.2:1 or 10.0 k

Ω:8.06 kΩ (using closest 1% values). Note

that for the lowest VOUT dc error, R2 R3 should be maintained

equal to R1 (as here), and the R2-R3 resistors should be stable,

close tolerance metal film types. The table in Figure 55 sum-

marizes R1-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-R8 divider. When the product of the output

current and the RS value match this voltage threshold, the cur-

rent 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 ILIMIT defined as:

ILIMIT =

VR(D2)









+ R8

()

27.4k

Ω

0.1µF

0.01µF

R11

Ω

4.99k

Ω

301k

Ω

+VS

R10

Ω

8.06k

Ω

0.1µF

0.05

Ω

10µF

4.99k

Ω

U1A

OP284

AD589

1N4148

22.1k

Ω

SI9433DY

2.21k

Ω

0.01µF

U1B

OP284

1N4148

REF192

1µF

4.53k

Ω

VOUT2

2.5V

10k

Ω

5.0V

4.5V

3.3V

3.0V

4.99k

4.53k

2.43k

1.69k

10.0k

8.06k

3.24k

2.00k

10.0k

VOUT R1

OUTPUT TABLE

VS > VOUT + 0.1V

OPTIONAL

ON/OFF CONTROL INPUT

CMOS HI (OR OPEN) = ON

LO = OFF

VIN COMMON

VOUT =

4.5V @ 350mA

(SEE TABLE)

VOUT COMMON

Figure 55. A Low Dropout Regulator with Current Limiting

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