MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
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where R
Pack
is the total resistance in the battery pack,
R
Board
is the board resistance in series with the battery
charge path, t
CV0
is the constant-voltage charge time
without remote sense, and t
CVRS
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:
V
IINP
= I
INPUT
x RS1 x G
IINP
x R8
where I
INPUT
is the DC current supplied by the AC
adapter, G
IINP
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 10kΩ 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.
V
DD
Supply
The V
DD
input provides power to the SMBus interface.
Connect V
DD
to LDO, or apply an external supply to
V
DD
to keep the SMBus interface active while the sup-
ply to DCIN is removed. When V
DD
is biased the inter-
nal registers are maintained. Bypass V
DD
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 V
CSSP
is less
than V
CSIN
+ 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).
•V
DD
Undervoltage (POR): When V
DD
is less than
2.5V, the V
DD
supply is in an undervoltage state and
the internal registers are in their POR state. The
SMBus interface does not respond to commands.
When V
DD
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 V
DD
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) V
CSSP
> V
CSIN
+ 210mV (15mV falling threshold)
3) V
DD
> 2.5V
