MAX1667
Chemistry-Independent,
Level 2 Smart Battery Charger
12 ______________________________________________________________________________________
_______________Detailed Description
Output Characteristics
The MAX1667 contains both a voltage-regulation loop
and a current-regulation loop. Both loops operate inde-
pendently of each other. The voltage-regulation loop
monitors BATT to ensure that its voltage never exceeds
the voltage set point (V0). The current-regulation loop
monitors current delivered to BATT to ensure that it
never exceeds the current-limit set point (I0). The cur-
rent-regulation loop is in control as long as BATT volt-
age is below V0. When BATT voltage reaches V0, the
current loop no longer regulates, and the voltage-regu-
lation loop takes over. Figure 5 shows the V-I character-
istic at the BATT pin.
Setting V0 and I0
Set the MAX1667’s voltage and current-limit set points
via the Intel SMBus 2-wire serial interface. The
MAX1667’s logic interprets the serial-data stream from
the SMBus interface to set internal digital-to-analog con-
verters (DACs) appropriately. The power-on-reset value
for V0 and I0 is 18.4V and 7mA, respectively. See Digital
Section for more information.
_____________________Analog Section
The MAX1667 analog section consists of a current-
mode pulse-width-modulated (PWM) controller and two
transconductance error amplifiers—one for regulating
current and the other for regulating voltage. The device
uses DACs to set the current and voltage level, which
are controlled via the SMBus interface. Since separate
amplifiers are used for voltage and current control, both
control loops can be compensated separately for opti-
mum stability and response in each state.
Whether the MAX1667 is controlling the voltage or cur-
rent at any time depends on the battery’s state. If the
battery has been discharged, the MAX1667’s output
reaches the current-regulation limit before the voltage
limit, causing the system to regulate current. As the bat-
tery charges, the voltage rises until the voltage limit is
reached, and the charger switches to regulating voltage.
The transition from current to voltage regulation is done
by the charger and need not be controlled by the host.
Figure 6 shows the MAX1667 block diagram.
Voltage Control
The internal GMV amplifier controls the MAX1667’s out-
put voltage. The voltage at the amplifier’s noninverting
input is set by an 11-bit DAC, which is controlled by a
ChargingVoltage() command on the SMBus (see Digital
Section for more information). The battery voltage is fed
to the GMV amplifier through a 5:1 resistive voltage
divider. The set voltage ranges between 0 and 18.416V
with 16mV resolution. This allows up to four Li+ cells in
series to be charged.
The GMV amplifier’s output is connected to the CCV
pin, which compensates the voltage-regulation loop.
Typically, a series-resistor/capacitor combination can
be used to form a pole-zero doublet. The pole intro-
duced rolls off the gain starting at low frequencies. The
zero of the doublet provides sufficient AC gain at mid-
frequencies. The output capacitor then rolls off the mid-
frequency gain to below 1 to guarantee stability before
encountering the zero introduced by the output capaci-
tor’s equivalent series resistance (ESR). The GMV
amplifier’s output is internally clamped to between one-
fourth and three-fourths of the voltage at REF.
Current Control
An internal 7mA linear current source is used in con-
junction with the PWM regulator to set the battery
charge current. When the current is set to 0, the voltage
regulator is on but no current is available. A current set-
ting between 1mA and 127mA turns on the linear cur-
rent source, providing a maximum of 7mA for trickle
charging. For current settings above 127mA, the linear
current source is disabled and the charging current is
provided by the switching regulator set by the 5-bit cur-
rent-control DAC.
The GMI amplifier’s noninverting input is driven by a 4:1
resistive voltage divider, which is driven by the 5-bit
DAC. With the internal 4.096V reference, this input is
approximately 1.0V at full scale, and the resolution is
31mV. The current-sense amplifier drives the inverting
input to the GMI amplifier. It measures the voltage
