ADA4870 datasheet(21/25 Pages) AD

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ADA4870

Data Sheet

Rev. 0 | Page 20 of 24

Figure 71. Small Signal Bandwidth for Various CL and RS Values from Figure 70

HEAT AND THERMAL MANAGEMENT

High output current amplifiers like the ADA4870 generate heat,

instantaneous or continuous, depending on the signal being

processed. Properly applied thermal management techniques

move heat away from the ADA4870 die and help to maintain

acceptable junction temperatures (TJ). A highly conductive

thermal path from the slug of the PSOP_3 package to the

ambient air is required to obtain the best performance at the

lowest TJ.

POWER DISSIPATION

The first step in identifying a thermal solution is to compute the

power generated in the amplifier during normal operation. The

schematic in Figure 72 shows a simplified output stage of the

ADA4870. The most significant heat is generated by the output

stage push-pull pair, particularly when driving heavy loads.

Figure 72. Simplified Output Stage

The total power dissipation in the amplifier is the sum of the

power dissipated in the output stage plus the quiescent power.

The average power for an amplifier processing sine signals is

computed by Equation 1. Equation 2 can be used to compute the

peak power of a sine wave and can be used to compute the

continuous power dissipation of dc output voltages where VPEAK is

the dc load voltage. These equations assume symmetrical supplies

and a load referred to midsupply.

(

)

























PEAK

SINE

AVG

–

(1)

(

) (

)













PEAK

–

(2)

where

VS is the total supply voltage (VCC – VEE).

Iq is the amplifier quiescent current.

A graphical representation of the PAVG,SINE and PPEAK power

equations is shown in Figure 73. The power curves were generated

for the ADA4870 operating from ±20 V supplies and driving a

20 Ω load. The quiescent power intersects the vertical axis at

~1.3 W when VOUT is at 0 V or midsupply. The graphs stop at

the output swing limit of 18 V.

For dc analysis, peak power dissipation occurs at VOUT = VCC/2,

while the maximum average power for sine wave signals occurs

at VOUT = 2VCC/π.

Figure 73. Average Sine and Peak Power vs. VOUT, VS = ±20 V, RL = 20 Ω

–15

–12

–9

–6

–3

0.1

100

FREQUENCY (MHz)

CL = 330pF, RS = 6.8Ω

CL = 1µF, RS = 0.3Ω

CL = 1nF, RS = 4Ω

CL = 3.3nF, RS = 2.5Ω

CL = 10nF, RS = 1.4Ω

CL = 33nF, RS = 0.7Ω

CL = 100nF, RS = 0.3Ω

GND

VOUT

VCC

VEE

VOUT (V)

PEAK (W)

AVG, SINE (W)

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