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Datasheet MAX1647 - Chemistry-Independent Battery Chargers - Maxim Integrated Products

Chemistry-Independent Battery ChargersMAX1647/MAX1648

Whether the MAX1647 is controlling the voltage or cur-rent at any time depends on the battery’s state. If thebattery has been discharged, the MAX1647’s outputreaches the current-regulation limit before the voltagelimit, causing the system to regulate current. As the bat-tery charges, the voltage rises until the voltage limit isreached, and the charger switches to regulating voltage.The transition from current to voltage regulation is doneby the charger, and need not be controlled by the host.

Voltage Control

The internal GMV amplifier controls the MAX1647’s out-put voltage. The voltage at the amplifier’s noninvertinginput amplifier is set by a 10-bit DAC, which is controlledby a ChargingVoltage( ) command on the SMBus (seethe MAX1647 Logicsection for more information). Thebattery voltage is fed to the GMV amplifier through a 4:1resistive voltage divider. With an external 4.096V refer-ence, the set voltage ranges between 0 and 16.38V with16mV resolution.

This poses a challenge for charging four lithium-ioncells in series: because the lithium-ion battery’s typicalper-cell voltage is 4.2V maximum, 16.8V is required. A larger reference voltage can be used to circumventthis. Under this condition, the maximum battery voltageno longer matches the programmed voltage. The solu-tion is to use a 4.2V reference and host software.Contact Maxim’s applications department for moreinformation.

The GMV amplifier’s output is connected to the CCVpin, which compensates the voltage-regulation loop.Typically, a series-resistor/capacitor combination canbe used to form a pole-zero couplet. The pole intro-duced rolls off the gain starting at low frequencies. Thezero 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 beforeencountering the zero introduced by the output capaci-tor’s equivalent series resistance (ESR). The GMVamplifier’s output is internally clamped to between one-fourth and three-fourths of the voltage at REF.

Figure 5. Output V-I Characteristic

Setting V0 and I0 (MAX1647)

Set the MAX1647’s voltage and current-limit set pointsvia the Intel System Management Bus (SMBus ) 2-wireserial 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.

Setting V0 and I0 (MAX1648)

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 fourtimes the voltage on the SETV pin. I0 equals the voltageon SETI divided by 5.5, divided by R1 (Figure 4).

_____________________Analog Section

The MAX1647/MAX1648 analog section consists of acurrent-mode PWM controller and two transconduc-tance error amplifiers: one for regulating current andthe other for regulating voltage. The MAX1647 usesDACs to set the current and voltage level, which arecontrolled 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). Sinceseparate amplifiers are used for voltage and currentcontrol, 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 fromthe MAX1647.

Current Control

The internal GMI amplifier and an internal currentsource control the battery current while the charger isregulating current. Since the regulator current’s accura-cy is not adequate to ensure full 11-bit accuracy, aninternal linear current source is used in conjunction withthe PWM regulator to set the battery current. The cur-rent-control DAC’s five least significant bits set the

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