OP484FS datasheet(18/24 Pages) AD

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OP184/OP284/OP484

Rev. D | Page 18 of 24

Second, the op amp amplifies the output signal to provide a rail-

to-rail output swing. In this particular case, the gain is set to 4.1

so that the circuit generates a 5 V output when the DAC output

is at full scale. If other output voltage ranges are needed, such as

0 V ≤ V

, C ), as shown in

A snubber consists of a series R-C network (RS

Figure 56, connected from the output of the device to ground.

This network operates in parallel with the load capacitor, CL, to

provide the necessary phase lag compensation. The value of the

resistor and capacitor is best determined empirically.

OUT

≤ 4.095 V, the gain can be easily changed by adjusting

the values of R2 and R3.

50Ω

0.1µF

1nF

100nF

VIN

100mV p-p

VOUT

1/2

OP284

HIGH-SIDE CURRENT MONITOR

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

design effort is focused on ensuring the long-term reliability a

of a pass transistor over a wide range of load current conditions.

As a result, monitoring and limiting device power dissipation is

of prime importance in these designs. The circuit illustrated in

Figure 55 is an example of a 3 V, single-supply, high-side current

monitor that can be incorporated into the design of a voltage

regulator with fold-back current limiting or a high current

power supply with crowbar protection. This design uses an

OP284’s rail-to-rail input voltage range to sense the voltage

drop across a 0.1 Ω current shunt. A P-channel MOSFET used

as the feedback element in the circuit converts the op amp’s

differential input voltage into a current. This current is applied

to R2 to generate a voltage that is a linear representation of the

load current. The transfer equation for the current monitor is

given by

Figure 56. Snubber Network Compensates for Capacitive Load

The first step is to determine the value of Resistor RS. A good

starting value is 100 Ω (typically, the optimum value is less than

100 Ω). This value is reduced until the small-signal transient

response is optimized. Next, CS is determined; 10 μF is a good

starting point. This value is reduced to the smallest value for

acceptable performance (typically, 1 μF). For the case of a 10 nF

load capacitor on the OP284, the optimal snubber network is a

20 Ω in series with 1 μF. The benefit is immediately apparent, as

shown in the scope photo in Figure 57. The top trace was taken

with a 1 nF load, and the bottom trace was taken with the 50 Ω,

100 nF snubber network in place. The amount of overshoot and

ringing is dramatically reduced.

SENSE

⎟

⎠

⎞

⎜

⎝

⎛

Monitor Output =

Table 6 shows a few sample

snubber networks for large load capacitors.

For the element values shown, the transfer characteristic of the

monitor output is 2.5 V/A.

2µs

100

50mV

1nF LOAD

ONLY

SNUBBER

CIRCUIT

DLY

5.49µs

50mV

RSENSE

0.1Ω

SI9433

MONITOR

OUTPUT

1/2

OP284

100Ω

2.49kΩ

0.1µF

Figure 57. Overshoot and Ringing Is Reduced by Adding a Snubber Network

in Parallel with the 1 nF Load

Figure 55. High-Side Load Current Monitor

Table 6. Snubber Networks for Large Capacitive Loads

CAPACITIVE LOAD DRIVE CAPABILITY

Load Capacitance (C

The OP284 exhibits excellent capacitive load driving capa-

bilities. It can drive up to 1 nF, as shown in Figure 28. Even

though the device is stable, a capacitive load does not come

without penalty in bandwidth. The bandwidth is reduced to less

than 1 MHz for loads greater than 2 nF. A snubber network

on the output does not increase the bandwidth, but it does

significantly reduce the amount of overshoot for a given

capacitive load.

)

Snubber Network (R , C )

1 nF

50 Ω, 100 nF

10 nF

20 Ω, 1 μF

100 nF

5 Ω, 10 μF

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