AD8450ASTZ datasheet(32/42 Pages) AD

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Data Sheet

AD8450

Rev. B | Page 31 of 41

Current Sensors

Two common options for current sensors are isolated current

sensing transducers and shunt resistors. Isolated current sensing

transducers are galvanically isolated from the power converter

and are affected less by the high frequency noise generated by

switch mode power supplies. Shunt resistors are less expensive

and easier to deploy.

If a shunt resistor sensor is used, a 4-terminal, low resistance shunt

resistor is recommended. Two of the four terminals conduct the

battery current, whereas the other two terminals conduct virtually

no current. The terminals that conduct no current are sense

terminals that are used to measure the voltage drop across the

resistor (and, therefore, the current flowing through it) using an

amplifier such as the PGIA of the AD8450. To interface the PGIA

with the current sensor, connect the sense terminals of the sensor

to the ISVP and ISVN pins of the AD8450 (see Figure 60).

Optional Low-Pass Filter

The AD8450 is designed to control both linear regulators and

switching power converters. Linear regulators are generally

noise free, whereas switch mode power converters generate

switching noise. Connecting an external differential low-pass

filter between the current sensor and the PGIA inputs reduces

the injection of switching noise into the PGIA (see Figure 60).

ISVP

ISVN

–

RGn

LPF

RGP

RGN

ISGPx

ISGNx

–

DUT

20kΩ

10kΩ

RFBP

RFBN

Figure 60. 4-Terminal Shunt Resistor Connected to the Current Sense PGIA

PGDA CONNECTIONS

For a description of the PGDA, see the Theory of Operation

section, Figure 49, and Figure 52. The internal gains of the

PGDA (0.2, 0.27, 0.4, and 0.8) are selected by connecting the

appropriate input pair to the battery terminals (see Table 6).

Table 6. PGDA Gain Connections

PGDA

Gain

Connect Battery

Positive Terminal to

Connect Battery

Negative Terminal to

0.8

BVP0 (Pin 25)

BVN0 (Pin 31)

0.4

BVP1 (Pin 24)

BVN1 (Pin 32)

0.27

BVP2 (Pin 23)

BVN2 (Pin 33)

0.2

BVP3 (Pin 22)

BVN3 (Pin 34)

Set the PGDA gain value to attenuate the voltage of up to four 5 V

battery cells in series to a full-scale voltage of 4 V. For example, a

5 V battery voltage is attenuated to 4 V using the gain of 0.8, and

a 20 V battery voltage (four 5 V batteries in series) is attenuated

to 4 V using the gain of 0.2. This voltage scaling enables the use of

a 5 V ADC to read the battery voltage at the BVMEA output pin.

Reverse Battery Conditions

The output voltage of the AD8450 PGDA can be used to detect

a reverse battery connection. A −5 V rail for AVEE allows the

output of the PGDA to go below ground when the battery is

connected backward. Therefore, the condition can be detected

by monitoring the BVMEA pin for a negative voltage.

BATTERY CURRENT AND VOLTAGE CONTROL

INPUTS (ISET AND VSET)

The voltages at the ISET and VSET input pins set the target

battery current and voltage for the constant current (CC) and

constant voltage (CV) loops. These inputs must be driven by a

precision voltage source (or a DAC connected to a precision

reference) whose output voltage is referenced to the same voltage

as the PGIA and PGDA reference pins (ISREFH/ISREFL and

BVREFH/BVREFL, respectively). For example, if the PGIA ref-

erence pins are connected to AGND, the voltage source connected

to ISET must also be referenced to AGND. In the same way, if

the PGDA 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

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

IBAT_SS =

ISET

where:

GIA is the PGIA gain.

RS is the value of the shunt resistor.

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

steady state, the VSET input sets the battery voltage as follows:

VBAT_SS =

VSET

where GDA is the PGDA gain.

Therefore, the accuracy and temperature stability of the formation

and test system are dependent not only on the precision of the

AD8450, but also on the accuracy of the ISET and VSET inputs.

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