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AD8451

Data Sheet

ISET

–

CC LOOP

AMPLIFIER

CV LOOP

AMPLIFIER

IVE1

VVE1

ANALOG

‘NOR’

ISVN

BVP

BVN

GDA

–

GIA

ISMEAS

BVMEA

IBAT

VSET

1×

VCTRL

VCLN

VCLP

VINT

BUFFER

VBAT

SENSE

RESISTOR

MODE

–

VINT

POWER

CONVERTER

VINT

IOUT

ISVP

VCTRL

CURRENT

POWER

BUS

MINIMUM

OUTPUT

SELECTOR

V3 < VCTRL < V4

Figure 46. Functional Block Diagram of the CC and CV Loops in Charge Mode (MODE Pin High)

The unity-gain amplifier (VINT buffer) buffers the VINT pins

and drives the VCTRL pin. The VCTRL pin is the control output

of the AD8451 and the control input of the power converter. The

VISET and VVSET voltage sources set the target constant current and

the target constant voltage, respectively. When the CC and CV

feedback loops are in a steady state, the charging current is set at

IBAT_SS =

ISET

where:

IBAT_SS = is the steady state charging current.

GIA is the IA gain.

RS is the value of the shunt resistor.

The target voltage is set at

VBAT_SS =

VSET

where:

VBAT_SS = steady state battery voltage.

GDA is the DA gain.

Because the offset voltage of the loop amplifiers is in series with

the target voltage sources, VISET and VVSET, the high precision of

these amplifiers minimizes this source of error.

Figure 47 shows a typical CC/CV charging profile for a Li-Ion

battery. In the first stage of the charging process, the battery is

charged with a CC of 1 A. When the battery voltage reaches a

target voltage of 4.2 V, the charging process transitions such that

the battery is charged with a CV of 4.2 V.

The following steps describe how the AD8451 implements the

CC/CV charging profile (see Figure 46). In this scenario, the

battery begins in the fully discharged state, and the system has

just been turned on such that IBAT = 0 A at Time 0.

1. Because the voltages at the ISMEA and BVMEA pins

are less than the target voltages (VISET and VVSET) at Time 0,

both integrators begin to ramp, increasing the voltage at

the VINT node.

1.25

0.25

0.50

0.75

1.00

TIME (Hours)

CHARGE

BEGINS

TRANSITION FROM CC TO CV

CHARGE

ENDS

Figure 47. Representative Constant Current to Constant Voltage Transition

near the End of a Battery Charging Cycle

2. As the voltage at the VINT node increases, the voltage at

the VCRTL node rises, and the output current of the power

converter, IBAT, increases (assuming that an increasing voltage

at the VCRTL node increases the output current of the

power converter).

3. When the IBAT current reaches the CC steady state value,

IBAT_SS, the battery voltage is still less than the target steady

state value, VBAT_SS. Therefore, the CV loop tries to keep

pulling the VINT node up while the CC loop tries to keep

it at its current voltage. At this point, the voltage at the ISMEA

pin equals VISET; therefore, the CC loop stops integrating.

4. Because the loop amplifiers can only pull the VINT node

down due to the analog NOR circuit, the CC loop takes

control of the charging feedback loop, and the CV loop is

disabled.

5. As the charging process continues, the battery voltage

increases until it reaches the steady state value, VBAT_SS, and

the voltage at the BVMEA pin reaches the target voltage, VVSET.

Rev. 0 | Page 20 of 32

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