ADD8708 datasheet(12/16 Pages) AD

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ADD8708

Rev. 0 | Page 12 of 16

OPERATING TEMPERATURE RANGE

The junction temperature is as follows:

TJ = TAMB + θJA × PDIS

where:

TAMB = ambient temperature.

θJA = junction-to-ambient thermal resistance, in °C/watt.

PDIS = power dissipated in the device, in watts.

For the ADD8708, PDIS can be calculated by this equation:

PDIS = VDD × IDQ + Σ(IOUT X(+) × (VDD − VOUT X)) +

Σ(−IOUT X(−) × VOUT X) + (VDD – VREG OUT ) × ILOAD

where:

VDD × IDQ = nominal system power requirements.

IOUT X(+) × (VDD − VOUT X)= positive-current amplifier load

power dissipation (current comes from VDD).

−IOUT X(−) × VOUT X = negative-current amplifier load power

dissipation (current goes to GND).

(VDD – VREG OUT) × ILOAD = regulator load power dissipation.

In this example, TAMB = 95°C. To calculate PDIS, assume the

values in Table 6.

Table 6.

VOUT X (V)

IOUT X (mA)

P (W)

VOUT18

14.400

4.3

0.00688

VOUT17

11.405

5.2

0.0239

VOUT16

10.627

−4.4

0.0468

VOUT15

10.397

7.3

0.0409

VOUT14

10.195

7.6

0.0441

VOUT13

10.080

−3.9

0.0393

VOUT12

9.821

8.3

0.0513

VOUT11

9.130

7.9

0.0543

VOUT10

8.611

−4.5

0.0389

VOUT9

6.480

−4.2

0.0272

VOUT8

6.077

5.6

0.0556

VOUT7

5.098

−3.3

0.0168

VOUT6

4.810

−6.9

0.0332

VOUT5

4.694

5.7

0.0644

VOUT4

4.435

3.5

0.0405

VOUT3

4.205

9.6

0.113

VOUT2

3.398

−9.5

0.0323

VOUT1

0.202

−7.2

0.00145

Σ(IOUT X(+) × (VDD

− VOUT X)) + Σ(−IOUT X(−) × VOUT X)

0.731

VDD × IDQ = 16 V × 15 mA = 0.240 W

(VDD – VREG OUT) × ILOAD = (16 V – 14.4 V) × 5 mA = 0.008 W

PDIS = 0.240 W + 0.731 W + 0.008 W =0.979 W

Example 1

Exposed pad soldered down with via θJA = 28.3°C/W:

TJ = 95°C + (28.3°C/W) × (0.979 W) = 122.7°C

where 150°C is the maximum junction temperature that is

guaranteed before the part breaks down. The maximum process

limit is 125°C. Because TJ is < 150°C and < 125°C, this example

demonstrates a condition where the part should perform within

process limits.

Example 2

Exposed pad not soldered down θJA = 47.7°C/W:

TJ = 95°C + (47.7°C/W) × (0.979 W) = 141.7°C

In this example, TJ is < 150°C but > 125°C. Although the part

should not exhibit any damage in this situation, the process

limits have been exceeded. The part may no longer operate

as intended.

These examples show that soldering down the exposed pad is

important for proper heat dissipation. Under the same power-

up and loading conditions, the unsoldered part has a higher

temperature than the soldered part. Therefore, it is strongly

advised that the exposed pad be soldered to VDD to maintain

part integrity.

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