OPA690IDBVT datasheet(21/37 Pages) TI1

21 / 37 page

OPA690

402W

175W

402W

+5V

50W

-5V

Power-supply

decouplingnotshown.

OPA690

www.ti.com

SBOS223F – DECEMBER 2001 – REVISED FEBRUARY 2010

OUTPUT CURRENT AND VOLTAGE

OPA690 can be very susceptible to decreased

stability and closed-loop response peaking when a

The OPA690 provides output voltage and current

capacitive load is placed directly on the output pin.

capabilities that are unsurpassed in a low-cost

When the amplifier's open-loop output resistance is

monolithic op amp. Under no-load conditions at

considered,

this

capacitive

load

introduces

+25°C, the output voltage typically swings closer than

additional pole in the signal path that can decrease

1V to either supply rail; the specified swing limit is

the phase margin. Several external solutions to this

within 1.2V of either rail. Into a 15

Ω load (the

problem have been suggested. When the primary

minimum tested load), it will deliver more than

considerations

are

frequency

response

flatness,

±160mA.

pulse response fidelity, and/or distortion, the simplest

and most effective solution is to isolate the capacitive

The

specifications

described

previously,

though

load

from

the

feedback

loop

inserting

familiar in the industry, consider voltage and current

series-isolation resistor between the amplifier output

limits separately. In many applications, it is the

and the capacitive load. This does not eliminate the

voltage × current, or V-I product, which is more

pole from the loop response, but rather shifts it and

relevant to circuit operation. Refer to Figure 19, the

adds a zero at a higher frequency. The additional

Output Voltage and Current Limitations plot in the

zero acts to cancel the phase lag from the capacitive

Typical Characteristics. The X- and Y-axes of this

load pole, thus increasing the phase margin and

graph show the zero-voltage output current limit and

improving stability.

the zero-current output voltage limit, respectively. The

four quadrants give a more detailed view of the

The Typical Characteristics show the recommended

OPA690 output drive capabilities, noting that the

RS versus capacitive load (Figure 15 for ±5V and

graph is bounded by a Safe Operating Area of 1W

Figure 30 for +5V) and the resulting frequency

maximum internal power dissipation. Superimposing

response at the load. Parasitic capacitive loads

resistor load lines onto the plot shows that the

greater

than

2pF

can

begin

degrade

the

OPA690 can drive ±2.5V into 25

Ω or ±3.5V into 50Ω

performance of the OPA690. Long PCB traces,

without exceeding the output capabilities or the 1W

unmatched cables, and connections to multiple

dissipation limit. A 100

Ω load line (the standard test

devices

can

easily

exceed

this

value.

Always

circuit load) shows the full ±3.9V output swing

consider

this

effect

carefully,

and

add

the

capability, as shown in the typical specifications.

recommended series resistor as close as possible to

the OPA690 output pin (see the Board Layout

The minimum specified output voltage and current

Guidelines section).

specifications over temperature are set by worst-case

simulations at the cold temperature extreme. Only at

The criterion for setting this RS resistor is a maximum

cold startup will the output current and voltage

bandwidth, flat frequency response at the load. For

decrease to the numbers shown in the Electrical

the OPA690 operating in a gain of +2, the frequency

Characteristic tables. As the output transistors deliver

response at the output pin is already slightly peaked

power,

their

junction

temperatures

increase,

without the capacitive load requiring relatively high

decreasing their VBEs (increasing the available output

values of RS to flatten the response at the load.

voltage swing) and increasing their current gains

Increasing the noise gain will reduce the peaking as

(increasing

the

available

output

current).

described

previously.

The

circuit

Figure

steady-state operation, the available output voltage

demonstrates this technique, allowing lower values of

and current is always greater than that shown in the

RS to be used for a given capacitive load.

over-temperature specifications because the output

stage junction temperatures will be higher than the

minimum specified operating ambient.

To protect the output stage from accidental shorts to

ground and the power supplies, output short-circuit

protection is included in the OPA690. The circuit acts

to limit the maximum source or sink current to

approximately 250mA.

DRIVING CAPACITIVE LOADS

One of the most demanding and yet very common

load conditions for an op amp is capacitive loading.

Often, the capacitive load is the input of an

ADC—including additional external capacitance which

may be recommended to improve ADC linearity. A

Figure 44. Capacitive Load Driving with Noise

high-speed, high open-loop gain amplifier like the

Gain Tuning

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