MAX1645B Datasheet, PDF (24/32 Pages) Maxim Integrated Products – Advanced Chemistry-Independent, Level 2 Battery Charger with Input Current Limiting

Advanced Chemistry-Independent, Level 2
Battery Charger with Input Current Limiting
Interrupts and Alert Response Address
The MAX1645B requests an interrupt by pulling the INT
pin low. An interrupt is normally requested when there is
a change in the state of the ChargerStatus() bits
POWER_FAIL (bit 13), BATTERY_PRESENT (bit 14), or
AC_PRESENT (bit 15). Therefore, the INT pin pulls low
whenever the AC adapter is connected or disconnect-
ed, the battery is inserted or removed, or the charger
goes in or out of dropout. The interrupts from each of
the ChargerStatus() bits can be masked by an associat-
ed ChargerMode() bit POWER_FAIL_MASK (bit 6), BAT-
TERY_PRESENT_MASK (bit 5), or AC_PRESENT_MASK
(bit 4).
All interrupts are cleared by sending any command to
the MAX1645B, or by sending a command to the
AlertResponse() address, 0x19, using a modified
receive-byte protocol. In this protocol, all devices that
set an interrupt try to respond by transmitting their
address, and the device with the highest priority, or
most leading zeros, are recognized and cleared. The
process repeats until all devices requesting interrupts
are addressed and cleared. The MAX1645B responds
to the AlertResponse() address with 0x13, which is its
address and a trailing 1.
Charger Timeout
The MAX1645B includes a timer that terminates charge
if the charger has not received a ChargingVoltage() or
ChargingCurrent() command in 175s. During charging,
the timer is reset each time a ChargingVoltage() or
ChargingCurrent() command is received; this ensures
that the charging cycle is not terminated.
If timeout occurs, charging terminates and both
ChargingVoltage() and ChargingCurrent() commands
are required to restart charging. A power-on reset also
restarts charging at 128mA.
DC-to-DC Converter
The MAX1645B employs a buck regulator with a boot-
strapped NMOS high-side switch and a low-side NMOS
synchronous rectifier.
DC-to-DC Controller
The control scheme is a constant off-time, variable-fre-
quency, cycle-by-cycle current mode. The off-time is
constant for a given BATT voltage; it varies with V_BATT
to keep the ripple current constant. During low-dropout
operation, a maximum on-time of 10ms allows the con-
troller to achieve >99% duty cycle with continuous con-
duction. Figure 8 shows the controller functional
diagram.
MOSFET Drivers
The low-side driver output DLO swings from 0V to DLOV.
DLOV is usually connected through a filter to LDO. The
high-side driver output DHI is bootstrapped off LX and
swings from V_LX to V_BST. When the low-side driver turns
on, BST rises to one diode voltage below DLOV.
Filter DLOV with an RC circuit whose cutoff frequency
is about 50kHz. The configuration in Figure 1 intro-
duces a cutoff frequency of around 48kHz:
f = 1 / 2πRC = 1 / (2 ✕ π ✕ 33Ω ✕ 0.1µF) = 48kHz
Thermistor Comparators
Four thermistor comparators evaluate the voltage at the
THM input to determine the battery temperature. This
input is meant to be used with the internal thermistor
connected to ground inside the battery pack. Connect
the output of the battery thermistor to THM. Connect a
resistor from THM to V_DD. The resistor-divider sets the
voltage at THM. When the charger is not powered up,
the battery temperature can still be determined if V_DD is
powered from an external voltage source.
Thermistor Bits
Figure 9 shows the expected electrical behavior of a
103ETB-type thermistor (nominally 10kΩ at +25°C ±5%
or better) to be used with the MAX1645B:
• THERMISTOR_OR bit is set when the thermistor
value is >100kΩ. This indicates that the thermistor is
open or a battery is not present. The charger is set to
POR, and the BATTERY_PRESENT bit is cleared.
• THERMISTOR_COLD bit is set when the thermistor
value is >30kΩ. The thermistor indicates a cold bat-
tery. This bit does not affect the charge.
• THERMISTOR_HOT bit is set when the thermistor
value is <3kΩ. This is a latched bit and is cleared by
removing the battery or sending a POR with the
ChargerMode() command. The MAX1645B charger
is stopped unless the HOT_STOP bit is cleared in the
ChargerMode() command or the RES_UR bit is set.
See Table 7.
• THERMISTOR_UR bit is set when the thermistor
value is <500Ω (i.e., THM is grounded).
Multiple bits can be set depending on the value of the
thermistor (e.g., a thermistor that is 450Ω causes both
the THERMISTOR_HOT and the THERMISTOR_UR bits
to be set). The thermistor can be replaced by fixed-
value resistors in battery packs that do not require the
thermistor as a secondary fail-safe indicator. In this
case, it is the responsibility of the battery pack to manip-
ulate the resistance to obtain correct charger behavior.
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