MAX1647 Maxim, MAX1647 Datasheet - Page 12

• Manufacturer Part Number: MAX1647
• Description: Chemistry-Independent Battery Chargers
• Manufacturer: Maxim

Set the MAX1647’s voltage and current-limit set points
via the Intel System Management Bus (SMBus™) 2-wire
serial interface. The MAX1647’s logic interprets the
serial-data stream from the SMBus interface to set inter-
nal digital-to-analog converters (DACs) appropriately.
See the MAX1647 Logic section for more information.
Set the MAX1648’s voltage- and current-limit set points
(V0 and I0, respectively) using external resistive dividers.
Figure 6b is the MAX1648 block diagram. V0 equals four
times the voltage on the SETV pin. I0 equals the voltage
on SETI divided by 5.5, divided by R1 (Figure 4).
The MAX1647/MAX1648 analog section consists of a
current-mode PWM controller and two transconduc-
tance error amplifiers: one for regulating current and
the other for regulating voltage. The MAX1647 uses
DACs to set the current and voltage level, which are
controlled via the SMBus interface. The MAX1648 elimi-
nates the DACs and controls the error amplifiers direct-
ly from SETI (for current) and SETV (for voltage). Since
separate amplifiers are used for voltage and current
control, both control loops can be compensated sepa-
rately for optimum stability and response in each state.
The following discussion relates to the MAX1647; how-
ever, MAX1648 operation can easily be inferred from
the MAX1647.
Chemistry-Independent
Battery Chargers
Figure 5. Output V-I Characteristic
_____________________Analog Section
12
V0
VOLTAGE
______________________________________________________________________________________
BATT
V0 = VOLTAGE SET POINT
I0 = CURRENT-LIMIT SET POINT
Setting V0 and I0 (MAX1647)
Setting V0 and I0 (MAX1648)
I0
AVERAGE CURRENT
THROUGH THE RESISTOR
BETWEEN CS AND BATT
Whether the MAX1647 is controlling the voltage or cur-
rent at any time depends on the battery’s state. If the
battery has been discharged, the MAX1647’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.
The internal GMV amplifier controls the MAX1647’s out-
put voltage. The voltage at the amplifier’s noninverting
input amplifier is set by a 10-bit DAC, which is controlled
by a ChargingVoltage( ) command on the SMBus (see
the MAX1647 Logic section for more information). The
battery voltage is fed to the GMV amplifier through a 4:1
resistive voltage divider. With an external 4.096V refer-
ence, the set voltage ranges between 0 and 16.38V with
16mV resolution.
This poses a challenge for charging four lithium-ion
cells in series: because the lithium-ion battery’s typical
per-cell voltage is 4.2V maximum, 16.8V is required. A
larger reference voltage can be used to circumvent
this. Under this condition, the maximum battery voltage
no longer matches the programmed voltage. The solu-
tion is to use a 4.2V reference and host software.
Contact Maxim’s applications department for more
information.
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 couplet. The pole intro-
duced rolls off the gain starting at low frequencies. The
zero of the couplet 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.
The internal GMI amplifier and an internal current
source control the battery current while the charger is
regulating current. Since the regulator current’s accura-
cy is not adequate to ensure full 11-bit accuracy, an
internal linear current source is used in conjunction with
the PWM regulator to set the battery current. The cur-
rent-control DAC’s five least significant bits set the
Voltage Control
Current Control