IRF7401 datasheet(18/22 Pages) AD

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REV.

–18–

ADN8830

The voltmeter to the TEC or output load should include the series

ammeter since the power delivered to the ammeter is considered part

of the total output power. However, the voltmeter measuring the

voltage delivered to the ADN8830 circuit should not include the

series ammeter from the power supply. This prevents a false supply

voltage power measurement since we are interested only in the

supply voltage power delivered to the ADN8830 circuit. Figures 16

and 17 show some efficiency measurements using the typical appli-

cation circuit shown in Figure 1.

ITEC (mA)

100

2,000

500

1,000

1,500

VSY = 5V

VSY = 3V

Figure 16. Efficiency with fCLK = 1 MHz

ITEC (mA)

100

2,000

500

1,000

1,500

VSY = 5V

VSY = 3V

Figure 17. Efficiency with fCLK = 200 kHz

Note that higher efficiency can be achieved using a lower supply

voltage or a slower clock frequency. This is due to the fact that the

dominant source of power dissipation at high clock frequencies is the

gate charge loss on the PWM transistors.

Layout Considerations

The two key considerations for laying out the board for the

ADN8830 are to minimize both the series resistance in the output

and the potential noise pickup in the precision input section. The

best way to accomplish both of these objectives is to divide the

layout into two sections, one for the output components and the

other for the remainder of the circuit. These sections should have

independent power supply and ground current paths that are each

connected together at a single point near the power supply. This is

used to minimize power supply and ground voltage bounce on the

more sensitive input stages to the ADN8830 caused by the switch-

ing of the PWM output. Such a layout technique is referred to as a

“star” ground and supply connection. Figure 18 shows a block dia-

gram of the concept.

VDD

POWER SUPPLY

GND

TEC

LOAD

OUTPUT

SECTION

NOISE

SENSITIVE

SECTION

PGND

PVDD

AGND

AVDD

Figure 18. Using Star Connections to Minimize

Noise Pickup from Switched Output

The low noise power and ground are referred to as AVDD and

AGND, with the output supply and ground paths labeled PVDD

and PGND. These pins are labeled on the ADN8830 and should

be connected appropriately. Both sets of external FETs should be

connected to PVDD and PGND. All output filtering and PVDD

supply bypass capacitors should be connected to PGND.

All remaining connections to ground and power supply should be

done through AVDD and AGND. A 4-layer board layout is rec-

ommended for best performance with split power and ground

planes between the top and bottom layers. This provides the

lowest impedance for both supply and ground points. Setting the

ADN8830 above the AGND plane will reduce the potential noise

injection into the device. Figure 19 shows the top layer of the

layout used for the ADN8830 evaluation boards, highlighting the

power and ground split planes.

Figure 19. Top Layer Reference Layout for ADN8830

Proper supply voltage bypassing should also be taken into consid-

eration to minimize the ripple voltage on the power supply. A

minimum bypass capacitance of 10

μF should be placed in close

proximity to each component connected to the power supply. This

includes Pins 8 and 20 on the ADN8830 and both external PMOS

transistors. An additional 0.1

μF capacitor should be placed in

parallel to each 10

μF capacitor to provide bypass for high fre-

quency noise. Using a large bulk capacitor, 100

μF or greater, in

parallel with a low ESR capacitor where AVDD and PVDD con-

nect will further improve voltage supply ripple. This is covered in

more detail in the Power Supply Ripple section.

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