AD8452 datasheet(20/35 Pages) AD

20 / 35 page

Data Sheet

AD8452

Rev. 0 | Page 19 of 34

BATTERY

VCTRL

CURRENT

SENSE

SHUNT

ISVP

ISVN

BVP

BVN

MODE

SWITCHES

(3)

BATTERY

CURRENT

(IBAT)

AVEE

CONSTANT CURRENT

LOOP FILTER AMPLIFIER

1×

VINT

BUFFER

ISET

VINT

ISMEA

BVMEA

AD8452

CONTROLLER

SET

BATTERY

VOLTAGE

SET

BATTERY

CURRENT

C = CHARGE

D = DISCHARGE

CONSTANT VOLTAGE

LOOP FILTER AMPLIFIER

BUCK

BOOST

PWM

SELECT

IBAT

POLARITY

1.1mA

VREG

CLP

CLN

CURRENT

LIMIT AND

DIODE

EMULATION

CHARGE

DISCHARGE

1×

DC TO DC

POWER CONVERSION

COMPENSATION MATRIX

IN-AMP

DIFF AMP

Figure 34. Signal Path of a Li-Ion Battery Formation and Test System Using the AD8452

INSTRUMENTATION AMPLIFIER (IN-AMP)

Figure 35 is a block diagram of the AD8452 in-amp used to

monitor battery current when connected to a low ohmic value

shunt. The architecture of the in-amp is the classic 3-op-amp

topology, similar to the Analog Devices, Inc., industry-standard

AD8221 or AD620, and is configured for a fixed gain of 66 V/V.

This architecture, combined with ADI exclusive precision laser

trimming, provides the highest achievable CMRR and optimizes

error free (gain error better than 0.1%) high-side battery

current sensing. For more information about instrumentation

amplifiers, see A Designer's Guide to Instrumentation Amplifiers.

10kΩ

20kΩ

500Ω

INSTRUMENTATION AMPLIFIER

G = 66

ISVN

ISVP

+ CURRENT

SHUNT

ISMEA

– CURRENT

SHUNT

SUBTRACTOR

100kΩ

19.5kΩ

ISREFH

ISREFL

2.5V

VREF

CHARGE/

DISCHARGE

POLARITY

INVERTER

10kΩ

305Ω

OPTIONAL CONNECTION FOR VOS OF 12.5mV

G = 2

+/–

Figure 35. Simplified Block Diagram of the Precision 3-Op-Amp In-Amp

Reversing Polarity When Charging and Discharging

Figure 34 shows that during the charge cycle, the power converter

drives current into the battery, generating a positive voltage

across the current sense shunt. During the discharge cycle,

however, the power converter drains current from the battery,

generating a negative voltage across the shunt resistor. In other

words, the battery current reverses polarity when the battery

discharges.

When in the constant current discharge mode control loop, this

reversal of the in-amp output voltage drives the integrator to the

negative rail unless the polarity of the target current is reversed.

To solve this problem, the AD8452 in-amp includes a double

pole, double throw switch preceding its inputs that implements

an input polarity inversion, thus correcting the sign of the

output voltage (see Figure 33). This multiplexer is controlled via

the MODE pin. When the MODE pin is logic high (charge

mode), the in-amp gain is noninverting, and when the MODE

pin is logic low (discharge mode), the in-amp gain is inverting.

The polarity control of the current sense voltage to the input of

the in-amp enables the integrator output voltage (VINT) to

always swing positive, regardless of the polarity of the battery

current.

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