MAX1645B 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 MAX1645B consists of current-sense amplifiers, an
SMBus interface, transconductance amplifiers, reference
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 current set by
the SMBus interface. The MAX1645B 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 cur-
rent-regulation loop monitors current delivered to BATT to
ensure that it never exceeds the current-limit set point
(I0). The battery current-regulation 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 battery-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 MAX1645B 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 MAX1645B ignores the first 4
LSBs and uses the next 11 LSBs to control the voltage
DAC. All codes greater than or equal to 0x4800
(18432mV) result in a voltage overrange, limiting the
charger voltage to 18.432V. All codes below 0x0400
(1024mV) terminate charging.
Setting the Charge Current
The MAX1645B charge-current DAC has a 3.2mV to
150.4mV range. The SMBus specification allows for a
16-bit ChargingCurrent() command that translates to a
0.05mV LSB and a 3.376V full-scale current-sense volt-
age. The MAX1645B drops the first 6 LSBs and uses
the remaining 6 MSBs to control the charge-current
DAC. All codes above 0x0BC0 result in an overrange
condition, limiting the charge current-sense voltage to
150.4mV. All codes below 0x0080 turn off the charging
current. Therefore, the charging current (I_CHARGE) is
determined by:
I_CHARGE = V_DACI / R_CSI
where V_DACI is the current-sense voltage set by
ChargingCurrent(), and R_CSI is the battery current-
sense resistor (R2 in Figure 1). When using a 50mΩ
current-sense resistor, the ChargingCurrent() value cor-
responds directly to the charging current in milliamps
(0x0400 = 1024mA = 52.2mV/50mΩ).
Input Current Limiting
The MAX1645B 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, CSS, compares the
voltage between CSSP and CSSN to the voltage at
CLS/20. V_CLS 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:
I_SOURCE = I_LOAD + [(I_CHARGE ✕ V_BATT) / (V_IN ✕ 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:
I_MAX = V_CLS / (20 ✕ R₁)
(Limit V_CSSP - V_CSSN to between 102.4mV and
204.8mV.)
The configuration in Figure 1 provides an input current
limit of:
I_MAX = (2.048V / 20) / 0.04Ω = 2.56A
LDO Regulator
An integrated LDO regulator provides a +5.4V supply
derived from DCIN, which 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 actu-
ally powered by DLOV and BST, which must be con-
nected to LDO through a lowpass filter and a diode as
shown in Figure 1. Also see the MOSFET Drivers sec-
tion. The LDO also supplies the 4.096V reference and
most of the control circuitry. Bypass LDO with a 1µF
capacitor.
V_DD Supply
This input provides power to the SMBus interface and
the thermistor comparators. Typically connect V_DD to
LDO or, to keep the SMBus interface of the MAX1645B
active while the supply to DCIN is removed, connect an
external supply to V_DD.
______________________________________________________________________________________ 11