MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 19

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

where R
R
charge path, t
without remote sense, and t
age charge time with remote sense.
The MAX8731 includes a safety feature, which limits the
charge voltage when FBS_ or the selector is discon-
nected. The MAX8731 guarantees that CSIN does not
regulate more than 200mV above the selected charg-
ing voltage. This also limits the extent to which remote
sense can cancel charge-path impedance.
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:
where I
adapter, G
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.
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 MAX8731 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.
The MAX8731 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.
Board
is the board resistance in series with the battery
INPUT
Pack
IINP
V
IINP
is the total resistance in the battery pack,
CV0
is the DC current supplied by the AC
is the transconductance of IINP (3mA/V
= I
SMBus Level 2 Battery Charger with
______________________________________________________________________________________
is the constant-voltage charge time
INPUT
Input Current Measurement
x RS1 x G
AC Adapter Detection
CVRS
is the constant-volt-
IINP
LDO Regulator
x R8
The V
Connect V
V
ply to DCIN is removed. When V
nal registers are maintained. Bypass V
a 0.1µF or greater ceramic capacitor.
The MAX8731 has the following operating states:
• Adapter Present: When DCIN is greater than 7.5V,
• Adapter Absent (Power Fail): When V
• V
The MAX8731 allows charging under the following conditions:
1) DCIN > 7.5V, LDO > 4V, REF > 3.1V
2) V
3) V
DD
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 MAX8731 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.
than V
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 MAX8731
quiescent battery current is less than 1µA (max).
2.5V, the V
the internal registers are in their POR state. The
SMBus interface does not respond to commands.
When V
power-on reset state. Charging does not occur until
the ChargeVoltage() and ChargeCurrent() com-
mands are sent. When V
SMBus registers are preserved.
DD
to keep the SMBus interface active while the sup-
CSSP
DD
DD
Undervoltage (POR): When V
> 2.5V
CSIN
input provides power to the SMBus interface.
DD
> V
DD
DD
+ 10mV, the MAX8731 is in the power-fail
CSIN
rises above 2.5V, the MAX8731 is in a
to LDO, or apply an external supply to
Remote Sense
supply is in an undervoltage state and
+ 210mV (15mV falling threshold)
Operating Conditions
DD
DD
is greater than 2.5V,
is biased the inter-
DD
DD
V
DD
to GND with
CSSP
is less than
Supply
is less
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