OPA621 datasheet(12/17 Pages) TI1

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OPA621

when driving the signal-dependent capacitances at the inputs

of flash A/D converters. As shown in Figure 3, the OPA621

maintains very low closed-loop output impedance over

frequency. Closed-loop output impedance increases with

frequency since loop gain is decreasing with frequency.

When the output is shorted to ground PDL = 5V x 150mA =

750mW. Thus, P

D = 280mW + 750mW = 1W. Note that the

short-circuit condition represents the maximum amount of

internal power dissipation that can be generated. Thus, the

“Maximum Power Dissipation” curve starts at 1W and is

derated based on a 175

°C maximum junction temperature

and the junction-to-ambient thermal resistance,

JA, of each

package. The variation of short-circuit current with tempera-

ture is shown in Figure 5.

FIGURE 3. Small-Signal Output Impedance vs Frequency.

THERMAL CONSIDERATIONS

The OPA621 does not require a heat sink for operation in

most environments. The use of a heat sink, however, will

reduce the internal thermal rise and will result in cooler,

more reliable operation. At extreme temperatures and under

full load conditions a heat sink is necessary. See “Maximum

Power Dissipation” curve, Figure 4.

FIGURE 4. Maximum Power Dissipation.

The internal power dissipation is given by the equation PD =

DQ + PDL, where PDQ is the quiescent power dissipation and

PDL is the power dissipation in the output stage due to the

load. (For

±V

CC = ±5V, PDQ = 10V x 28mA = 280mW,

max). For the case where the amplifier is driving a grounded

load (RL) with a DC voltage (±VOUT) the maximum value of

PDL occurs at ±VOUT = ±VCC/2, and is equal to PDL, max =

(

±V

CC)

2/4R

L. Note that it is the voltage across the output

transistor, and not the load, that determines the power

dissipated in the output stage.

FIGURE 5. Short-Circuit Current vs Temperature.

CAPACITIVE LOADS

The OPA621’s output stage has been optimized to drive

resistive loads as low as 50

Ω. Capacitive loads, however,

will decrease the amplifier’s phase margin which may cause

high frequency peaking or oscillations. Capacitive loads

greater than 15pF should be buffered by connecting a small

resistance, usually 5

Ω to 25Ω, in series with the output as

shown in Figure 6. This is particularly important when

driving high capacitance loads such as flash A/D converters.

In general, capacitive loads should be minimized for opti-

mum high frequency performance. Coax lines can be driven

if the cable is properly terminated. The capacitance of coax

cable (29pF/foot for RG-58) will not load the amplifier

when the coaxial cable or transmission line is terminated in

its characteristic impedance.

FIGURE 6. Driving Capacitive Loads.

OPA621

(R typically 5

to 25 )

ΩΩ

100

10k

100k

10M

100M

Frequency (Hz)

0.1

0.01

G = +10V/V

G = +2V/V

G = +5V/V

1.2

1.0

0.8

0.6

0.4

0.2

+25

+50

+75

+100

+125

+150

Ambient Temperature (°C)

Plastic, SO-8

Packages

250

200

150

100

–75

–50

–25

+25

+50

+75

+100

+125

Ambient Temperature (°C)

– I

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