8731AE datasheet(19/32 Pages) MAXIM

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SMBus Level 2 Battery Charger

with Remote Sense

______________________________________________________________________________________

where RPack is the total resistance in the battery pack,

RBoard is the board resistance in series with the battery

charge path, tCV0 is the constant-voltage charge time

without remote sense, and tCVRS is the constant-volt-

age charge time with remote sense.

The MAX8731A includes a safety feature, which limits

the charge voltage when FBS_ or the selector is dis-

connected. The MAX8731A guarantees that CSIN does

not regulate more than 250mV above the selected

charging voltage. This also limits the extent to which

remote sense can cancel charge-path impedance.

Input Current Measurement

Use IINP to monitor the system-input current sensed

across CSSP and CSSN. The voltage at IINP is propor-

tional to the input current by the equation:

VIINP = IINPUT x RS1 x GIINP x R8

where IINPUT is the DC current supplied by the AC

adapter, GIINP is the transconductance of IINP (3mA/V

typ), and R8 is the resistor connected between IINP

and ground. Typically, IINP has a 0 to 3.5V output volt-

age range. Leave IINP open if not used.

LDO Regulator

An integrated low-dropout (LDO) linear regulator pro-

vides a 5.4V supply derived from DCIN, and delivers over

30mA of load current. The LDO powers the gate drivers

of the n-channel MOSFETs. See the MOSFET Drivers

section. LDO has a minimum current limit of 35mA. This

allows the MAX8731A to work with 87nC of total gate

charge (both high-side and low-side MOSFETs). Bypass

LDO to PGND with a 1µF or greater ceramic capacitor.

AC Adapter Detection

The MAX8731A includes a hysteretic comparator that

detects the presence of an AC power adapter. When

ACIN is greater than 2.048V, the open-drain ACOK out-

put becomes high impedance. Connect 10k

Ω pullup

resistance between LDO and ACOK. Use a resistive

voltage-divider from the adapter’s output to the ACIN

pin to set the appropriate detection threshold. Select

the resistive voltage-divider not to exceed the 6V

absolute maximum rating of ACIN.

VDD Supply

The VDD input provides power to the SMBus interface.

Connect VDD to LDO, or apply an external supply to

VDD to keep the SMBus interface active while the sup-

ply to DCIN is removed. When VDD is biased the inter-

nal registers are maintained. Bypass VDD to GND with

a 0.1µF or greater ceramic capacitor.

Operating Conditions

The MAX8731A has the following operating states:

• Adapter Present: When DCIN is greater than 7.5V,

the adapter is considered to be present. In this con-

dition, both the LDO and REF function properly and

battery charging is allowed:

a) Charging: The total MAX8731A quiescent current

when charging is 1mA (max) plus the current required

to drive the MOSFETs.

b) Not Charging: To disable charging, set either

ChargeCurrent() or ChargeVoltage() to zero. When the

adapter is present and charging is disabled, the total

adapter quiescent current is less than 1mA and the

total battery quiescent current is less than 5µA.

• Adapter Absent (Power Fail): When VCSSP is less

than VCSIN + 10mV, the MAX8731A is in the power-

fail state, since the DC-DC converter is in dropout.

The charger does not attempt to charge in the

power-fail state. Typically, this occurs when the

adapter is absent. When the adapter is absent, the

total MAX8731A quiescent battery current is less

than 1µA (max).

•VDD Undervoltage (POR): When VDD is less than

2.5V, the VDD supply is in an undervoltage state and

the internal registers are in their POR state. The

SMBus interface does not respond to commands.

When VDD rises above 2.5V, the MAX8731A is in a

power-on reset state. Charging does not occur until

the ChargeVoltage() and ChargeCurrent() com-

mands are sent. When VDD is greater than 2.5V,

SMBus registers are preserved.

The MAX8731A allows charging under the following

conditions:

1) DCIN > 7.5V, LDO > 4V, REF > 3.1V

2) VCSSP > VCSIN + 210mV (15mV falling threshold)

3) VDD > 2.5V

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