MAX1645-MAX1645A Maxim, MAX1645-MAX1645A Datasheet - Page 11

• Manufacturer Part Number: MAX1645-MAX1645A
• Description: Advanced Chemistry-Independent / Level 2 Battery Chargers with Input Current Limiting
• Manufacturer: Maxim

The MAX1645/MAX1645A consist 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 volt-
age and the current set by the SMBus interface. The
MAX1645/MAX1645A feature 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 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 cur-
rent limit.
The MAX1645/MAX1645A voltage DACs have a 16mV
LSB and an 18.432V full scale. The SMBus specifica-
tion allows for a 16-bit ChargingVoltage() command
that translates to a 1mV LSB and a 65.535V full-scale
voltage; therefore, the ChargingVoltage() value corre-
sponds to the output voltage in millivolts. The
MAX1645/MAX1645A ignore the first four LSBs and use
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.
The MAX1645/MAX1645A current DACs have 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 cur-
rent; the ChargingCurrent() value corresponds to the
charging voltage in milliamps. The MAX1645/
MAX1645A drop the first six LSBs and use 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 current 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/cur-
rent scaling.
Battery Chargers with Input Current Limiting
Advanced Chemistry-Independent, Level 2
______________________________________________________________________________________
Detailed Description
Setting Output Voltage
Setting Output Current
The MAX1645/MAX1645A limit the current drawn by the
charger when the load current becomes high. The
devices limit 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
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:
where η is the efficiency of the DC-DC converter (typi-
cally 85% to 95%).
V
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:
(Limit V
204.8mV.)
The configuration in Figure 1 provides an input current
limit of:
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. See also 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.
This input provides power to the SMBus interface and
the thermistor comparators. Typically connect V
LDO or, to keep the SMBus interface of the
MAX1645/MAX1645A active while the supply to DCIN is
removed, connect an external supply to V
CLS
I
SOURCE
determines the threshold voltage of the CSS com-
CSSP
I
MAX
= I
= (2.048V / 20) / 0.04Ω = 2.56A
LOAD
- V
I
MAX
CSSN
+ [(I
= V
CLS
CHARGE
CLS
to between 102.4mV and
Input Current Limiting
is set by a resistor-divider
/ (20 · R
· V
BATT)
LDO Regulator
1
)
V
DD
/ (V
DD
.
IN
Supply
· η)]
DD
11
to