AD8450ASTZ datasheet(28/41 Pages) AD

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AD8450

Data Sheet

Rev. B | Page 28 of 41

CURRENT SHARING BUS AND IMAX OUTPUT

Battery formation and test systems that use multiple channels

bridged together to condition high current battery cells require

circuitry to balance the total output current among the channels.

Current balance, or current sharing (CS), can be implemented

by actively matching the output current of each channel during

the battery charge/discharge process.

The current sharing bus amplifier is a precision unity-gain

specialty amplifier with an output stage that can only pull up its

output node (the CSH pin). The amplifier is configured as a

unity-gain buffer with its input connected to the ISMEA pin

(the output of the PGIA). If the CSH pin is left unconnected, the

voltage at the pin is a replica of the voltage at the ISMEA pin.

Figure 58 is a functional block diagram of the current sharing

circuit. In this example, Channel 0 through Channel n are

bridged together to charge a high current battery.

The CS output of each channel is tied to a common bus (CS bus),

which is buffered by the CS buffer amplifier to the IMAX pin.

By means of external resistors, the uncommitted operational

amplifier is configured as a difference amplifier to measure the

voltage difference between the IMAX and ISMEA nodes.

During the charge process, the charging current and, therefore,

the voltage at the ISMEA pin, is slightly different in each channel

due to mismatches in the components that make up each channel.

Because the CS bus amplifiers are driven by their respective PGIAs

and have output stages that can only pull up their output nodes,

the amplifier that requires the highest voltage takes control of the

CS bus. Therefore, the voltage at the CS bus is pulled up to match

the VISMEA voltage of the channel with the largest output current.

The output voltage of the uncommitted op amp in each channel

is proportional to the difference between the channel’s output

current and the largest output current. This output voltage can

then be used to form a feedback loop that actively corrects the

channel’s output current by adjusting the channel’s target current

and target voltage, that is, adjusting VISET and VVSET voltages.

I_n

CHANNEL n

CHANNEL 1

IMAX

ISVP

ISVN

–

ISMEA

PGIA

OAVO

OAVN

RS0

OAVP

BUFFER

CS BUS

AMPLIFIER

–

CHANNEL 0

AVEE

UNCOMMITTED

OP AMP

CORRECTION

SIGNAL

VCS − VISMEA

CHANNEL 0

I_0

I_1

CS BUS

IBAT

I_0

Figure 58. Functional Block Diagram of the Current Sharing Circuit

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