OP481GS datasheet(12/16 Pages) AD

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OP181/OP281/OP481

–12–

REV. 0

+3V

OP281-A

VIN

Ω

5.1k

Ω

+3V

VOUT

5.1k

Ω

10k

Ω

+3V

OP281-B

Figure 39. Using the OP281 as a Window Comparator

The threshold limits for the window are set by VH and VL,

provided that VH > VL. The output of A1 will stay at the

negative rail, in this case ground, as long as the input voltage is

less than VH. Similarly, the output of A2 will stay at ground as

long the input voltage is higher than VL. As long as VIN remains

between VL and VH, the outputs of both op amps will be 0 V.

With no current flowing in either D1 or D2, the base of Q1 will

stay at ground, putting the transistor in cutoff and forcing VOUT

to the positive supply rail. If the input voltage rises above VH,

the output of A2 stays at ground, but the output of A1 will go to

the positive rail, and D1 will conduct current. This creates a

base voltage that will turn on Q1 and drive VOUT low. The same

condition occurs if VIN falls below VL with A2’s output going

high, and D2 conducting current. Therefore, VOUT will be high

if the input voltage is between VL and VH, and VOUT will be low

if the input voltage moves outside of that range.

The R1 and R2 voltage divider sets the upper window voltage,

and the R3 and R4 voltage divider sets the lower voltage for the

window. For the window comparator to function properly, VH

must be a greater voltage than VL.

VH =

+ R2

VL =

+ R4

The 2 k

Ω resistor connects the input voltage to the input termi-

nals to the op amps. This protects the OP281 from possible

excess current flowing into the input stages of the devices. D1

and D2 are small-signal switching diodes (1N4446 or equiva-

lent), and Q1 is a 2N2222 or equivalent NPN transistor.

A Low-Side Current Monitor

In the design of power supply control circuits, a great deal of

design effort is focused on ensuring a pass transistor’s long-term

reliability over a wide range of load current conditions. As a

result, monitoring and limiting device power dissipation is of

prime importance in these designs. Figure 40 shows an example

of a +5 V, single-supply current monitor that can be incorpo-

rated into the design of a voltage regulator with fold-back

current limiting or a high current power supply with crowbar

protection. The design capitalizes on the OP181’s common-

mode range that extends to ground. Current is monitored in the

power supply return path where a 0.1

Ω shunt resistor, R

SENSE,

creates a very small voltage drop. The voltage at the inverting

terminal becomes equal to the voltage at the noninverting

terminal through the feedback of Q1, which is a 2N2222 or

equivalent NPN transistor. This makes the voltage drop across

R1 equal to the voltage drop across RSENSE. Therefore, the

current through Q1 becomes directly proportional to the current

through RSENSE, and the output voltage is given by:

VOUT = VEE −

× R

SENSE × I L









The voltage drop across R2 increases with IL increasing, so

VOUT decreases with higher supply current being sensed. For

the element values shown, the VOUT transfer characteristic is

–2.5 V/A, decreasing from VEE.

+5V

RETURN TO

GROUND

OP181

+5V

2.49k

Ω

VOUT

100

Ω

0.1

Ω

RSENSE

Figure 40. A Low-Side Load Current Monitor

Low Voltage Half-Wave and Full-Wave Rectifiers

Because of its quick overdrive recovery time, an OP281 can be

configured as a full-wave rectifier for low frequency (<500 Hz)

applications. Figure 41 shows the schematic.

+3V

OP281-A

VIN = 2V p-p

Ω

+3V

OP281-B

R1 = 100k

Ω

R2 = 100k

Ω

FULL-WAVE

RECTIFIED

OUTPUT

HALF-WAVE

RECTIFIED

OUTPUT

Figure 41. Single Supply Full- and Half-Wave Rectifiers

Using an OP281

100

SCALE 0.1V/DIV

SCALE 0.1ms/DIV

Figure 42. Full-Wave Rectified Signal

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