OPA690 datasheet(21/27 Pages) TI

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OPA2613

SBOS249D − JUNE 2003− REVISED APRIL 2004

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OUTPUT CURRENT AND VOLTAGE

The OPA2613 provides output voltage and current

capabilities that are unsurpassed in a low-cost dual

monolithic op amp. Under no-load conditions at 25

°C, the

output voltage typically swings closer than 1V to either

supply rail; tested at +25

°C, swing limit is within 1.1V of

either rail. Into a 12

Ω load (the minimum tested load), it

delivers more than

±280mA continuous output current.

The specifications described previously, though familiar in

the industry, consider voltage and current limits separately.

In many applications, it is the voltage times current (or V-I

product) that is more relevant to circuit operation. Refer to

the Output Voltage and Current Limitations plot in the

Typical Characteristics. The X and Y axes of this graph

show the zero-voltage output current limit and the

zero-current output voltage limit, respectively. The four

quadrants give a more detailed view of the OPA2613

output drive capabilities, noting that the graph is bounded

by a safe operating area of 1W maximum internal power

dissipation

(in

this

case,

for one channel only).

Superimposing resistor load lines onto the plot shows that

the OPA2613 can drive +4.8 and −4.1 into 25

Ω without

exceeding the output capabilities or the 1W dissipation

limit. A 100

Ω load line (the standard test circuit load)

shows the full

±4.9V output swing capability, as shown in

the Electrical Characteristics tables. The minimum

specified output voltage and current over temperature are

set by worst-case simulations at the cold temperature

extreme. Only at cold startup will the output current and

voltage decrease to the numbers shown in the Electrical

Characteristics tables. As the output transistors deliver

power, the junction temperatures increase, decreasing the

VBEs (increasing the available output voltage swing), and

increasing the current gains (increasing the available

output current). In steady-state operation, the available

output voltage and current will always be greater than that

shown in the over-temperature specifications, since the

output stage junction temperatures will be higher than the

minimum specified operating ambient.

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 that may be recom-

mended to improve the ADC linearity. A high-speed, high

open-loop gain amplifier like the OPA2613 can be very

susceptible to decreased stability and closed-loop

response peaking when a capacitive load is placed directly

on the output pin. When the amplifier open-loop output

resistance is considered, this capacitive load introduces

an additional pole in the signal path that can decrease the

phase margin. Several external solutions to this problem

have been suggested.

When the primary considerations are frequency response

flatness, pulse response fidelity, and/or distortion, the

simplest and most effective solution is to isolate the

capacitive load from the feedback loop by inserting a

series isolation resistor between the amplifier output and

the capacitive load. This does not eliminate the pole from

the loop response, but rather shifts it and adds a zero at a

higher frequency. The additional zero acts to cancel the

phase lag from the capacitive load pole, thus increasing

the phase margin and improving stability. The Typical

Characteristics show the Recommended RS vs Capacitive

Load and the resulting frequency response at the load.

Parasitic capacitive loads greater than 2pF can begin to

degrade the performance of the OPA2613. Long PC board

traces, unmatched cables, and connections to multiple

devices can easily cause this value to be exceeded.

Always consider this effect carefully, and add the

recommended series resistor as close as possible to the

OPA2613 output pin (see the Board Layout Guidelines

section).

The very high output current and unity gain stability for the

OPA2613 can be used to drive large capacitive loads with

moderate slew rates. An example is shown in Figure 11

where a 5000pF load cap is driven with a 1MHz square

wave to give a

±5V swing. The supplies were slightly

increased to give more headroom for the charging current

through the 2

Ω isolation resistor.

1/2

OPA2613

402

Ω

+6.2V

Supply decoupling

not shown.

−6.2V

Ω

402

Ω

5000pF

±2.5V

1MHz

Square

Wave

Input

Figure 11. Large Capacitive Load Driver

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