MAX1645EEI Maxim, MAX1645EEI Datasheet - Page 26

• Manufacturer Part Number: MAX1645EEI
• Description: Advanced Chemistry-Independent / Level 2 Battery Chargers with Input Current Limiting
• Manufacturer: Maxim

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.
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
voltage at THM. When the charger is not powered up,
the battery temperature can still be determined if V
powered from an external voltage source.
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
• THERMISTOR_COLD bit is set when the thermistor
• THERMISTOR_HOT bit is set when the thermistor
Figure 9. Typical Thermistor Characteristics
26
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.
value is >30kΩ. The thermistor indicates a cold bat-
tery. This bit does not affect the charge.
value is <3kΩ. This is a latched bit and is cleared by
______________________________________________________________________________________
f = 1 / 2 RC = 1 / (2 ·
1000
100
0.1
10
1
-50
-40
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110
DD
Thermistor Comparators
TEMPERATURE (°C)
. The resistor-divider sets the
· 33Ω · 0.1µF) = 48kHz
Thermistor Bits
DD
is
• THERMISTOR_UR bit is set when the thermistor
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.
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-
• The load switch P2 is controlled by PDL. This P-
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) .
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.
value is <500Ω (i.e., THM is grounded).
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 .
channel MOSFET is turned off when the CVS rises to
100mV below BATT and turns on when CVS falls to
300mV below BATT.
Load and Source Switch Drivers
Dropout Operation