ADA4870 datasheet(22/25 Pages) AD

22 / 25 page

Data Sheet

ADA4870

Rev. 0 | Page 21 of 24

SAFE OPERATING AREA

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

output 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 ≤ 150°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.

Two heat sinks (VHS-45 and VHS-95) were used in the evaluation.

Both were assembled to the PCB using CT40-5 thermal interface

material.

All testing was done in a still-air environment. Forced air

convection in any of the test cases effectively lowers θJA and

moves the corresponding curve toward the upper right,

expanding the SOA. For more information on the ADA4870

evaluation board, see the ADA4870 User Guide.

In Figure 74, the horizontal line at 1 A is the output current

drive of the ADA4870. The curved section maintains a fixed

power dissipation that results in a junction temperature (TJ) of

150°C or less. Note that the x-axis is the output stage VCE (VCC −

VOUT or VOUT – VEE) developed across the relevant output

transistor of Figure 72 and ends at a maximum VCE of 20 V.

Figure 74. Safe Operating Area for Evaluation Board from Figure 75 at 25°C

and 85°C Ambient Temperature With and Without Heat Sink, No Air Flow

Figure 75. Details of the ADA4870 Evaluation Board

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

OUTPUT STAGE VCE (V)

MAXIMUM TJ = 150°C

25°C WITH VHS-45

25°C NO HEATSINK

85°C WITH VHS-95

85°C WITH VHS-45

85°C NO HEATSINK

6-LAYER HR370 PCB

WITH INTERNAL GROUND

AND POWER PLANES

COPPER

TOP/BOTTOM: 1.5oz

INNER LAYERS: 1oz

EXPOSED PAD LANDING VIAS

COUNT: 136

FILL: AE3030

DIAMETER: 12mil

PITCH: 35mil

2.45in (62mm)

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