Data Sheet

ADA4870

APPLICATIONS INFORMATION

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Rev. B | 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 exposed pad of the LFCSP

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 sink presented in the

Safe Operating Area section and Figure 74 is 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.

θHS= TJ–TAPDISS – θJC+θCB+θTIM

(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 ca-

pacitors 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) at

TA = 25°C

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

Not applicable

Not applicable

0

16.3

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

Not applicable

Not applicable

2.73

11.43

61 × 58 × 24

2.29

8

0

9.94

61 × 58 × 24

2.29

8

2.73

7.26