AD8452 datasheet(31/35 Pages) AD

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AD8452

Data Sheet

Rev. 0 | Page 30 of 34

reference pins are connected to AGND, the voltage source

connected to VSET must also be referenced to AGND.

In constant current mode, when the CC feedback loop is in a

steady state, the ISET input sets the battery current as follows:

IBAT =

ISET

where:

GIA is the in-amp gain.

RS is the value of the current sense resistor.

Note that the system accuracy is highly dependent on the physical

properties of the shunt as well as the in-amp GIA and VISET values.

When selecting a shunt, be sure to consider temperature

performance as well as basic precision.

In constant voltage mode, when the CV feedback loop is in a

steady state, the VSET input sets the battery voltage according

to the equation:

VBAT =

VSET

where GDA is the difference amplifier gain.

Therefore, the accuracy and temperature stability of the formation

and test system are not only dependent on the accuracy and

stability of the AD8452 but also on the accuracy external

components.

LOOP FILTER AMPLIFIERS

The AD8452 has two loop filter amplifiers, also known as error

amplifiers (see Figure 49). One amplifier is for constant current

control (CC loop filter amplifier), and the other amplifier is for

constant voltage control (CV loop filter amplifier). The outputs

of these amplifiers are combined using a minimum output

selector circuit to perform automatic CC to CV switching.

Table 9 lists the inputs of the loop filter amplifiers for charge

mode and discharge mode.

Table 9. Integrator Input Connections

Feedback Loop Function

Reference

Input

Feedback

Terminal

Control the Current While Discharging

a Battery

ISET

IVE0

Control the Current While Charging

a Battery

ISET

IVE1

Control the Voltage While Discharging

a Battery

VSET

VVE0

Control the Voltage While Charging

a Battery

VSET

VVE1

The CC and CV amplifiers in charge mode and the CC amplifier in

discharge mode are inverting integrators, whereas the CV amplifier

in discharge mode is a noninverting integrator. Therefore, the

CV amplifier in discharge mode uses an extra amplifier, the VSET

buffer, to buffer the VSET input pin (see Figure 33). In addition,

the CV amplifier in discharge mode uses the VVP0 pin to couple

the signal from the BVMEA pin to the integrator.

SELECTING CHARGE OR DISCHARGE OPTIONS

To operate the AD8452 in discharge mode (including energy

recycling) mode, apply a voltage less than 1.05 V (typical) to the

MODE pin. To operate the AD8452 in charge mode, drive the

MODE pin high, greater than 1.20 V (typical). The state of the

MODE pin can change when the AD8452 is shut down via the EN

pin or via an external fault condition signaled on the FAULT

pin, a TSD event, or an UVLO condition.

SELECT RCL AND RCLVT FOR THE PEAK CURRENT

LIMIT

Figure 47 is the block diagram for peak current limit and diode

emulation. Note that the current-limit sense resistor is floating

between the output filter inductor and capacitor.

The current generated by a fault condition defines the peak current.

In turn, the peak current equals the sum of the average current

(rated battery charging or discharging current) and the peak

incremental inductor current:

IPK = IAVG + IMAX

where:

IAVG is the battery charge/discharge current.

ILMAX is the the inductor saturation current.

Typically, the peak current level is set to the sum of the average

current and the value of the inductor saturation current.

Use the following equation to calculate the minimum current-limit

sense resistor value:

RCL MIN (Ω) =

)

(

(2)

where:

IPK is the desired peak current limit in A.

RCL MIN is the minimum current limit sense resistor value in Ω.

50 mV is the minimum IR drop across RCL for sufficient noise

immunity during operation.

Select the next higher standard resistor value for RCL.

Next, the value for RCLVT is calculated using the following

equation:

RCLVT (Ω) =













CLVT

(3)

where:

RCLVT is the current-limit threshold voltage resistor value in Ω.

IPK is the desired peak current limit in A.

RCL is the current-limit sense resistor value in Ω.

ICLVT is the CLVT pin current (21 µA typical)

The AD8452 is designed so that the peak current limit is the

same in both the buck mode and the boost mode of operation. A

1% or better tolerance for the RCL and RS resistors is recommended.

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