Data Sheet

ADA4870

APPLICATIONS INFORMATION

analog.com

Rev. B | 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 proc-

essed. 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 exposed pad of the LFCSP 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.

PAVG,SINE= VS×Iq + 2π×VCCVPEAK

RL – VPEAK22RL (1)

PPEAK= VS×Iq + VCC– VPEAK × VPEAKRL

(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

SAFE OPERATING AREA

The safe operating area (SOA) is a curve of output current vs. out-

put stage collector-emitter voltage (VCE), under which the amplifier

can operate at a safe junction temperature (TJ). The area under

the curves of Figure 74 shows the operational boundaries of the

ADA4870 for the PCB of Figure 75 that maintains a TJ ≤ 140°C.

The SOA curves of Figure 74 are unique to the conditions under

which they were developed, such as PCB, heat sink, and ambient

temperature.

Wakefield-Vette 518-95AB heat sink was used in the evaluation. It

was assembled to the PCB using GC Electronics 10-8109 Type Z9

thermal interface material.