MAX1644 Datasheet, PDF (11/32 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
Detailed Description
The MAX1645 consists of current-sense amplifiers, an
SMBus interface, transconductance amplifiers, refer-
ence circuitry, and a DC–DC converter (Figure 2). The
DC–DC converter generates the control signals for the
external MOSFETs to maintain the voltage and the cur-
rent set by the SMBus interface. The MAX1645 features
a voltage-regulation loop and two current-regulation
loops. The loops operate independently of each other.
The voltage-regulation loop monitors BATT to ensure
that its voltage never exceeds the voltage set point
(V0). The battery current-regulation loop monitors cur-
rent delivered to BATT to ensure that it never exceeds
the current-limit set point (I0). The battery current-regu-
lation loop is in control as long as BATT voltage is
below V0. When BATT voltage reaches V0, the current
loop no longer regulates. A third loop reduces the bat-
tery-charging current when the sum of the system (the
main load) and the battery charger input current
exceeds the charging source current limit.
Setting Output Voltage
The MAX1645’s voltage DAC has a 16mV LSB and an
18.432V full scale. The SMBus specification allows for a
16-bit ChargingVoltage() command that translates to a
1mV LSB and a 65.535V full-scale voltage; therefore,
the ChargingVoltage() value corresponds to the output
voltage in millivolts. The MAX1645 ignores the first four
LSBs and uses the next 11 LSBs to control the voltage
DAC. All codes greater than or equal to 0b0100 1000
0000 0000 (18432mV) result in a voltage overrange,
limiting the charger voltage to 18.432V. All codes below
0b0000 0100 0000 0000 (1024mV) terminate charging.
Setting Output Current
The MAX1645’s current DAC has a 64mA LSB and a
3.008A full scale. The SMBus specification allows for a
16-bit ChargingCurrent() command that translates to a
1mA LSB and a 65.535A full-scale current; the
ChargingCurrent() value corresponds to the charging
voltage in milliamps. The MAX1645 drops the first six
LSBs and uses the next six LSBs to control the current
DAC. All codes above 0b00 1011 1100 0000 (3008mA)
result in a current overrange, limiting the charger cur-
rent to 3.008A. All codes below 0b0000 0000 1000
0000 (128mA) turn the charging current off. A 50mΩ
sense resistor (R2 in Figure 1) is required to achieve
the correct CODE/current scaling.
Input Current Limiting
The MAX1645 limits the current drawn by the charger
when the load current becomes high. The device limits
the charging current so the AC adapter voltage is not
loaded down. An internal amplifier compares the volt-
age between CSSP and CSSN to the voltage at CLS/20.
VCLS is set by a resistor divider between REF and
GND.
The input source current is the sum of the device cur-
rent, the charge input current, and the load current. The
device current is minimal (6mA max) in comparison to
the charge and load currents. The charger input cur-
rent is generated by the DC-DC converter; therefore, the
actual source current required is determined as follows:
ISOURCE = ILOAD + [(ICHARGE · VBATT) / (VIN · η)]
where η is the efficiency of the DC-DC converter (typi-
cally 85% to 95%).
V_CLS determines the threshold voltage of the CSS com-
parator. R3 and R4 (Figure 1) set the voltage at CLS.
Sense resistor R1 sets the maximum allowable source
current. Calculate the maximum current as follows:
IMAX = VCLS / (20 · R1)
(Limit VCSSP - VCSSN to between 102.4mV and
204.8mV.)
The configuration in Figure 1 provides an input current
limit of:
IMAX = (2.048V / 20) / 0.04Ω = 2.56A
LDO Regulator
The LDO provides a +5.4V supply derived from DCIN
and can deliver up to 15mA of current. The LDO sets
the gate-drive level of the NMOS switches in the
DC-DC converter. The drivers are actually powered by
DLOV and BST, which must be connected to LDO
through a lowpass filter and a diode as shown in Figure
1. See also the MOSFET Drivers section. The LDO also
supplies the 4.096V reference and most of the control
circuitry. Bypass LDO with a 1µF capacitor.
VDD Supply
This input provides power to the SMBus interface and
the thermistor comparators. Typically connect VDD to
LDO or, to keep the SMBus interface of the MAX1645
active while the supply to DCIN is removed, connect an
external supply to VDD.
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