AD8451 datasheet(26/33 Pages) AD

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Data Sheet

AD8451

Step 4: Determine the Control Voltage for the CC Loop

and the Shunt Resistor

The relationship between the control voltage for the CC loop

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

IA gain is as follows:

CC Battery Target Current =

ISET

The voltage across the shunt resistor is as follows:

Shunt Resistor Voltage =

ISET

For target current of 1 A, choosing a 20 mΩ shunt resistor

results in a control voltage of 4 V.

When selecting a shunt resistor, consider the resistor style and

construction. For low power dissipation applications, many

temperature stable SMD styles can be soldered to a heat sink

pad on a printed circuit board (PCB). For optimum accuracy,

choose a shunt resistor that provides force and sense terminals.

In these resistors, the battery current flows through the force

terminals and the voltage drop in the resistor is read at the sense

terminals.

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 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 IA and

DA. For example, if the IA and DA 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 references available about how to design the compensation

for such power converters. The AD8451 provides active loop

filter error amplifiers for the CC and CV control loops that

can implement proportional integrator (PI), PD, and PID

compensators using external passive components.

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