AD8451 datasheet(18/33 Pages) AD

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Data Sheet

AD8451

THEORY OF OPERATION

OVERVIEW

To form and test a battery, the battery must undergo charge and

discharge cycles. During these cycles, the battery terminal current

and voltage must be precisely controlled to prevent battery failure

or a reduction in the capacity of the battery. Therefore, battery

formation and test systems require a high precision analog front

end to monitor the battery current and terminal voltage. The

analog front end of the AD8451 includes a precision current

sense fixed gain instrumentation amplifier (IA) to measure the

battery current and a precision voltage sense fixed gain difference

amplifier (DA) to measure the battery voltage.

Battery formation and test systems charge and discharge batteries

using a constant current/constant voltage (CC/CV) algorithm. In

other words, the system first forces a set constant current into

or out of the battery until the battery voltage reaches a target

value. At this point, a set constant voltage is forced across the

battery terminals.

The AD8451 provides two control loops—CC loop and a CV

loop—that transition automatically after the battery reaches the

user defined target voltage. These loops are implemented via two

precision specialty amplifiers with external feedback networks

that set the transfer function of the CC and CV loops.

Moreover, in the AD8451, these loops reconfigure themselves to

charge or discharge the battery by toggling the MODE pin.

Figure 42 is a block diagram of the AD8451 that illustrates

the distinct sections of the AD8451, including the IA and DA

measurement blocks, and the loop filter amplifiers. Figure 43 is a

block diagram of a battery formation and test system.

ISVP

MODE

RGP

RGN

ISVN

–

AVEE

1×

VCTRL

VCLN

VVE1

VINT

VCLP

VVE0

VVP0

VSET

VSETBF

VREF

AVCC

DVCC

DGND

VINT

BUFFER

VSET

BUFFER

10kΩ

20kΩ

100Ω

80kΩ

+/–

10kΩ

1667Ω

10kΩ

CV LOOP

FILTER

AMPLIFIER

CC LOOP

FILTER

AMPLIFIER

BATTERY

CURRENT

SENSING IA

BATTERY

VOLTAGE

SENSING

CONSTANT

CURRENT AND

VOLTAGE LOOP

FILTER AMPLIFIERS

–

AD8451

1.1mA

2.5V

VREF

ISREFL

BUFFER

1×

Figure 42. Detailed Block Diagram

Rev. 0 | Page 17 of 32

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