OP184/OP284/OP484

Rev. J | Page 20 of 24

Obviously, it is desirable to keep this comparison voltage small

because it becomes a significant portion of the overall dropout

voltage. Here, the 20 mV reference is higher than the typical

offset of the OP284 but is still reasonably low as a percentage

of VOUT (<0.5%). In adapting the limiter for other ILIMIT levels,

Sense Resistor RS should be adjusted along with R7 to R8, to

maintain this threshold voltage between 20 mV and 50 mV.

Performance of the circuit is excellent. For the 4.5 V output

version, the measured dc output change for a 225 mA load

change was on the order of a few microvolts, while the dropout

voltage at this same current level was about 30 mV. The current

limit, as shown in Figure 58, is 400 mA, allowing the circuit to

be used at levels up to 300 mA or more. While the Q1 device can

actually support currents of several amperes, a practical current

rating takes into account the 2.5 W, 25°C dissipation of the

8-lead SOIC device. Because a short-circuit current of 400 mA

at an input level of 5 V causes a 2 W dissipation in Q1, other input

conditions must be considered carefully in terms of potential

overheating of Q1. Of course, if higher powered devices are used

for Q1, this circuit can support outputs of tens of amperes as

well as the higher VOUT levels already noted.

The circuit shown can either be used as a standard low dropout

regulator, or it can be used with on/off control. By driving Pin 3

of U2 with the optional logic control signal, VC, the output is

switched between on and off. Note that when the output is off

in this circuit, it is still active (that is, not an open circuit). This

is because the off state simply reduces the voltage input to R1,

leaving the U1A/U1B amplifiers and Q1 still active.

When the on/off control is used, Resistor R10 should be used

with U2 to speed on/off switching and to allow the output of the

circuit to settle to a nominal zero voltage. Component D3 and

Component R11 also aid in speeding up the on/off transition by

providing a dynamic discharge path for C2. Off/on transition

time is less than 1 ms, while the on/off transition is longer, but

less than 10 ms.

3 V, 50 HZ/60 HZ ACTIVE NOTCH FILTER WITH

FALSE GROUND

To process signals in a single-supply system, it is often best to use

a false ground biasing scheme. A circuit that uses this approach is

shown in Figure 59. In this circuit, a false ground circuit biases

an active notch filter used to reject 50 Hz/60 Hz power line

interference in portable patient monitoring equipment.

Notch filters are commonly used to reject power line frequency

interference that often obscures low frequency physiological

signals, such as heart rates, blood pressure readings, EEGs, and

EKGs. This notch filter effectively squelches 60 Hz pickup at a

Filter Q of 0.75. Substituting 3.16 kΩ resistors for the 2.67 kΩ

resistor in the twin-T section (R1 through R5) configures the

active filter to reject 50 Hz interference.

R2

2.67k

Ω

R6

10k

Ω

R7

1k

Ω

R8

1k

Ω

R11

10k

Ω

R9

20k

Ω

R12

150

Ω

R10

20k

Ω

1

3

5

6

7

11

2

3V

VO

VIN

A2

A1

8

A3

4

10

9

3V

A1, A2, A3 = OP484

Q = 0.75

NOTE: FOR 50Hz APPLICATIONS

CHANGE R1, R2, R3, AND R4 TO 3.1k

Ω

AND R5 TO 1.58k

Ω (3.16kΩ ÷ 2).

R3

2.67k

Ω

R1

2.67k

Ω

R4

2.67k

Ω

R5

1.33k

Ω

(2.68k

Ω ÷ 2)

C3

2µF

(1µF × 2)

C5

0.03µF

C1

1µF

C2

1µF

C4

1µF

C6

1µF

1.5V

Figure 59. A 3 V Single-Supply, 50Hz to 60 Hz Active Notch Filter

with False Ground

Amplifier A3 is the heart of the false ground bias circuit. It buffers

the voltage developed at R9 and R10 and is the reference for the

active notch filter. Because the OP484 exhibits a rail-to-rail input

common-mode range, R9 and R10 are chosen to split the 3 V

supply symmetrically. An in-the-loop compensation scheme is

used around the OP484 that allows the op amp to drive C6, a

1 μF capacitor, without oscillation. C6 maintains a low impedance

ac ground over the operating frequency range of the filter.

The filter section uses an OP484 in a Twin-T configuration whose

frequency selectivity is very sensitive to the relative matching of

the capacitors and resistors in the twin-T section. Mylar is the

material of choice for the capacitors, and the relative matching

of the capacitors and resistors determines the pass band symmetry

of the filter. Using 1% resistors and 5% capacitors produces satis-

factory results.