MAX1644 Datasheet, PDF (26/32 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
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 VDD. The resistor-divider sets the
voltage at THM. When the charger is not powered up,
the battery temperature can still be determined if VDD 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 MAX1645:
• 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
1000
100
10
removing the battery or sending a POR with the
ChargerMode() command. The charger is stopped
unless the HOT_STOP bit is cleared in the
ChargerMode() command.
• THERMISTOR_UR bit is set when the thermistor
value is <500Ω (i.e., THM is grounded).
Multiple bits may be set depending on the value of the
thermistor (e.g., a thermistor that is 450Ω will cause
both the THERMISTOR_HOT and the THERMISTOR_UR
bits to be set). The thermistor may 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
manipulate the resistance to obtain correct charger
behavior.
Load and Source Switch Drivers
The MAX1645 can drive two P-channel MOSFETs to
eliminate voltage drops across the Schottky diodes,
which are normally used to switch the load current from
the battery to the main DC source:
• The source switch P1 is controlled by PDS. This P-
channel MOSFET is turned on when CVS rises to
300mV above BATT and turns off when CVS falls to
100mV above BATT. The same signal that controls
the PDS also sets the POWER_FAIL bit in the
Charger Status() register. See Operating Conditions.
• The load switch P2 is controlled by PDL. This P-
channel MOSFET is turned off when the CVS rises to
100mV below BATT and turns on when CVS falls to
300mV below BATT.
Dropout Operation
The MAX1645 has a 99.99% duty-cycle capability with
a 10ms maximum on-time and 1µs off-time. This allows
the charger to achieve dropout performance limited
only by resistive losses in the DC-DC converter compo-
nents (P1, R1, N1, R2; see Figure 1). The actual
dropout voltage is limited to 300mV between CVS and
BATT by the power-fail comparator (see Operating
Conditions).
1
0.1
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
TEMPERATURE (°C)
Figure 9. Typical Thermistor Characteristics
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