AD8450ASTZ datasheet(34/42 Pages) AD

34 / 42 page

Data Sheet

AD8450

Rev. B | Page 33 of 41

Step 4: Determine the Control Voltage for the CC Loop,

the Shunt Resistor, and the PGIA Gain

The relationship between the control voltage for the CC loop

(the voltage at the ISET pin), the target battery current, and the

PGIA gain is as follows:

CC Battery Target Current =

Gain

PGIA

ISET

The voltage across the shunt resistor is as follows:

Shunt Resistor Voltage =

Gain

PGIA

VISET

Selecting the highest PGIA gain of 200 reduces the voltage across

the shunt resistor, minimizing dissipated power and inaccuracies

due to self heating. For a PGIA gain of 200 and a target current

of 1 A, choosing a 20 mΩ shunt resistor results in a control

voltage of 4 V.

When selecting a shunt resistor, pay close attention to the

resistor style and construction. For low power applications,

many surface-mount (SMD), temperature stable styles are

available that solder to a mating pad on a printed circuit board

(PCB). For optimum accuracy, specify a 4-terminal shunt

resistor that provides separate high current and sense terminals.

This type of resistor directs the majority of the battery current

through a high current path. An additional pair of terminals

provides a separate connection for the input leads to the PGIA,

avoiding the power loss inherent to forcing the full battery

current through the distance to the PGIA pins. Because the bias

current is so low, the sense error is significantly less than if the

battery current were to transverse the additional lead length.

Step 5: Choose the Control Voltage Sources

The input control voltages (the voltages at the ISET and VSET pins)

can be generated by an analog voltage source such as a voltage

reference or by a digital-to-analog converter (DAC). In both

cases, select a device that provides a stable, low noise output

voltage. If a DAC is preferred, Analog Devices offers a wide

range of precision converters. For example, the AD5668 16-bit

DAC provides up to eight 0 V to 4 V sources when connected to

an external 2 V reference.

To maximize accuracy, the control voltage sources must be

referenced to the same potential as the outputs of the PGIA and

PGDA. For example, if the PGIA and PGDA reference pins are

connected to AGND, connect the reference pins of the control

voltage sources to AGND.

Step 6: Select the Compensation Devices

Feedback controlled switching power converters require frequency

compensation to guarantee loop stability. There are many refer-

ences available about how to design the compensation for such

power converters. The AD8450 provides active loop-filter error-

amplifiers for the CC and CV control loops that can implement

PI, PD, and PID compensators using external passive components.

ADDITIONAL INFORMATION

Additional information relative to using the AD8450 is available in

the AN-1319, Compensator Design for a Battery Charge/Discharge

Unit Using the AD8450 or the AD8451.

AD8450ASTZ Datenblatt(PDF) 34 Page - Analog Devices

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