OP484EP datasheet(16/20 Pages) AD

16 / 20 page

REV. 0

–16–

OP184/OP284/OP484

Obviously, it is desirable to keep this comparison voltage small,

since 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 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-R8, to main-

tain 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 is 400 mA, which allows 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 SO-8 device’s 2.5 W, 25

°C dissipation.

A short circuit current of 400 mA at an input level of 5 V will

cause a 2 W dissipation in Q1, so other input conditions should

be considered carefully in terms of Q1’s potential overheating.

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 noted above.

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

regulator, or it can also be used with ON/OFF control. By

driving Pin 3 of U1 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 (i.e., not an open cir-

cuit). This is because the OFF state simply reduces the voltage

input to R1, leaving the U1A/B amplifiers and Q1 still active.

When ON/OFF control is used, resistor R10 should be used

with U1, to speed ON-OFF switching, and to allow the output

of the circuit to settle to a nominal zero voltage. Components

D3 and 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 under 10 ms.

A +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 illustrated in Figure 56. 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 equip-

ment. Notch filters are quite commonly used to reject power

line frequency interference which often obscures low frequency

physiological signals, such as heart rates, blood pressure read-

ings, EEGs, EKGs, et cetera. This notch filter effectively

squelches 60 Hz pickup at a filter Q of 0.75. Substituting

3.16 k

Ω resistors for the 2.67 kΩ in the twin-T section

(R1 through R5) configures the active filter to reject 50 Hz

interference.

+3V

2.67k

Ω

1µF

2.67k

Ω

2µF

(1µF x 2)

2.67k

Ω

1.33k

Ω

(2.67k

Ω ÷ 2)

2.67k

Ω

10k

Ω

VIN

Ω

R11

10k

Ω

0.03µF

R12

150

Ω

R10

20k

Ω

1µF

20k

Ω

+3V

1.5V

1µF

A1, A2, A3 = OP484

Q = 0.75

NOTE: FOR 50Hz APPLICATIONS

CHANGE R1–R4 TO 3.1k

Ω

AND R5 TO 1.58k

Ω (3.16kΩ ÷ 2).

Figure 56. A +3 V Single Supply, 50/60 Hz Active Notch

Filter with False Ground

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

simply buffers the voltage developed at R9 and R10 and is the

reference for the active notch filter. Since 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 compen-

sation 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 a 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 filter’s pass band

symmetry. Using 1% resistors and 5% capacitors produces

satisfactory results.

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