ADA4870 datasheet(23/25 Pages) AD

23 / 25 page

ADA4870

Data Sheet

Rev. 0 | Page 22 of 24

PRINTED CIRCUIT BOARD (PCB)

All current feedback amplifiers, including the ADA4870, can be

affected by stray capacitance. Paying careful attention during

PCB layout can reduce parasitic capacitance and improve

overall circuit performance. Minimize signal trace lengths by

placing feedback and gain setting resistors as close as possible to

the amplifier.

Additionally, for high output current amplifiers like the

ADA4870, lay out the PCB with heat dissipation in mind. A

good thermal design includes an exposed copper landing area

on the top side of the board on which to solder the thermal slug

of the PSOP3 package. The PCB should also provide an exposed

copper area on the bottom side to accommodate a heat sink.

Stitch the top and bottom layers together with an array of

plated-through thermal vias to facilitate efficient heat transfer

through the board. Thermal conductivity may be further

improved by using widely available via fill materials.

THERMAL MODELING

Computational fluid dynamics (CFD) tools like FloTherm® can

be used to create layers of materials that include PCB construction,

thermal vias, thermal interface materials, and heat sinks, and

can predict junction temperature and/or junction to ambient

thermal resistance (θJA) for a given set of conditions. Table 7

shows an example of how θJA is affected by the addition of an

aluminum heat sink and forced convection. Figure 76 shows an

image of the model used to establish the thermal results in Table 7.

Figure 76. Thermal Model Stack-Up for Data in Table 7

(Heat Sink Not Shown)

HEAT SINK SELECTION

A heat sink increases the surface area to ambient temperature

(TA) and extends the power dissipation capability of the

ADA4870 and PCB combination. To maximize heat transfer

from the board to the heat sink, attach the heat sink to the PCB

using a high conductivity thermal interface material (TIM). The

heat sinks presented in the Safe Operating Area section and

Figure 74 are effective up to ~10 W in still air. If lower power

dissipation is anticipated and/or forced air convection is used, a

smaller heat sink may be appropriate. If the thermal resistance

of the chip (θJC), PCB (θCB), and TIM (θTIM) are known, use

Equation 3 to compute the thermal resistance (θHS) of the

required heat sink.

(

)

TIM

DISS













–

(3)

POWER SUPPLIES AND DECOUPLING

The ADA4870 can operate from a single supply or dual supplies.

The total supply voltage (VCC − VEE) must be between 10 V and

40 V. Decouple each supply pin to ground using high quality,

low ESR, 0.1 μF capacitors. Place decoupling capacitors as close

to the supply pins as possible. Additionally, place 22 μF tantalum

capacitors from each supply to ground to provide good low

frequency decoupling and supply the needed current to support

large, fast slewing signals at the ADA4870 output.

Table 7. Effects of Heat Sink and Forced Convection on θJA

Heat Sink Dimensions, L × W × Total Height (mm)

Heat Sink Base Thickness (mm)

No. of Fins

Air Flow (m/sec)

θJA (°C/W)

61 × 58, Exposed Copper on Board, No Heat Sink

Not applicable

15.95

61 × 58, Exposed Copper on Board, No Heat Sink

Not applicable

12.27

61 × 58, Exposed Copper on Board, No Heat Sink

Not applicable

10.95

30 × 30 × 24

11.36

30 × 30 × 24

4.90

30 × 30 × 24

3.86

61 × 58 × 24

5.74

61 × 58 × 24

3.59

61 × 58 × 24

3.18

TBOARD

TCASE

TPLASTIC

ADA4870

CU SLUG

TLEADS

TJUNCTION

DIE

ADA4870 Datasheet(PDF) 23 Page - Analog Devices

ADA4870 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Datasheet Download

Link URL