OP484FS datasheet(18/24 Pages) AD

18 / 24 page

OP184/OP284/OP484

Rev. J | Page 18 of 24

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 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 shown 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 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 differential input voltage of the op amp 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

Monitor Output =

SENSE













For the element values shown, the transfer characteristic of the

monitor output is 2.5 V/A.

RSENSE

0.1

Ω

SI9433

MONITOR

OUTPUT

1/2

OP284

100

Ω

2.49k

Ω

0.1µF

Figure 55. High-Side Load Current Monitor

CAPACITIVE LOAD DRIVE CAPABILITY

The OP284 exhibits excellent capacitive load driving capabilities.

It can drive up to 1 nF, as shown in Figure 30. 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.

A snubber consists of a series R-C network (RS, CS), as shown in

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.

Ω

0.1µF

1nF

100nF

VIN

100mV p-p

VOUT

1/2

OP284

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. Table 7 shows a few sample

snubber networks for large load capacitors.

2µs

100

50mV

1nF LOAD

ONLY

SNUBBER

CIRCUIT

DLY

5.49µs

50mV

Figure 57. Overshoot and Ringing Are Reduced by Adding a Snubber

Network in Parallel with the 1 nF Load

Table 7. Snubber Networks for Large Capacitive Loads

Load Capacitance (CL)

Snubber Network (RS, CS)

1 nF

50 Ω, 100 nF

10 nF

20 Ω, 1 µF

100 nF

5 Ω, 10 µF

OP484FS Datasheet(PDF) 18 Page - Analog Devices

OP484FS Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL