AD8451ASTZ datasheet(23/32 Pages) AD

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Data Sheet

AD8451

POWER SUPPLY CONNECTIONS

The AD8451 requires two analog power supplies (AVCC and

AVEE), one digital power supply (DVCC), one analog ground

(AGND), and one digital ground (DGND). AVCC and AVEE

power all the analog blocks, including the IA, DA, and op amps,

and DVCC powers the MODE input logic. AGND provides a

reference and return path for the 2.5 V reference, and DGND

provides a reference and return path for the digital circuitry.

The rated absolute maximum value for AVCC − AVEE is 36 V,

and the minimum operating AVCC and AVEE voltages are +5 V

and −5 V, respectively. Due to the high PSRR of the AD8451

analog blocks, AVCC can be connected directly to the high current

power bus (the input voltage of the power converter) without

risking the injection of supply noise to the controller outputs.

A commonly used power supply combination is +15 V for

AVCC, −15 V for AVEE, and +5 V for DVCC. The +15 V rail

for AVCC provides enough headroom to the IA such that it can

be connected in a high-side current sensing configuration. The

−15 V rail for AVEE allows the DA to sense accidental reverse

battery conditions (see the Reverse Battery Conditions section).

Connect decoupling capacitors to all the supply pins. A 1 µF

capacitor in parallel with a 0.1 µF capacitor is recommended.

CURRENT SENSE IA CONNECTIONS

For a description of the IA, see the Theory of Operation section,

Figure 42, and Figure 44. The IA fixed gain is 26.

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 IA of the AD8451. To interface the IA with

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

the ISVP and ISVN pins of the AD8451 (see Figure 50).

Optional Low-Pass Filter

The AD8451 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 IA inputs reduces the

injection of switching noise into the IA (see Figure 50).

ISVP

–

10kΩ

20kΩ

10kΩ

LPF

RGP

4 TERMINAL

SHUNT

–

DUT

ISVN

RGN

–

10kΩ

1667Ω

Figure 50. 4-Terminal Shunt Resistor Connected to the Current Sense IA

VOLTAGE SENSE DA CONNECTIONS

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

section, Figure 42, and Figure 45. The DA fixed gain is 0.8.

Reverse Battery Conditions

The output voltage of the AD8451 DA can be used to detect a

reverse battery connection. A −15 V rail for AVEE allows the

output of the DA 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 CC and CV loops. These

inputs must be driven by a precision voltage source (or a digital-

to-analog converter [DAC] connected to a precision reference)

whose output voltage is referenced to the same voltage as the IA

and DA reference pins (ISREFH/ISREFL and BVREFH/BVREFL,

respectively). For example, if the IA reference pins are connected

to AGND, the voltage source connected to ISET must also be

referenced to AGND. In the same way, if the DA 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_SS =

ISET

where:

GIA is the IA gain.

RS is the value of the shunt resistor.

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