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

Chemistry-Independent Battery ChargersMAX1647/MAX1648

The synchronous rectifier may not be completelyreplaced by a diode because the BST capacitorcharges while the synchronous rectifier is turned on.Without the synchronous rectifier, the BST capacitormay not fully charge, leaving the high-side MOSFETwith insufficient gate drive to turn on. However, the syn-chronous rectifier may be replaced with a small MOS-FET, such as a 2N7002, to guarantee that the BSTcapacitor is allowed to charge. In this case, most of thecurrent at high currents is carried by the diode and notby the synchronous rectifier.

Internal Regulator and Reference

The MAX1647 uses an internal low-dropout linear regula-tor to create a 5.4V power supply (VL), which powers itsinternal circuitry. VL can supply up to 20mA. A portion ofthis current powers the internal circuitry, but the remain-ing current can power the external circuitry. The currentused to drive the MOSFETs comes from this supply,which must be considered when calculating how muchpower can be drawn. To estimate the current required todrive the MOSFETs, multiply the total gate charge ofeach MOSFET by the switching frequency (typically250kHz). The internal circuitry requires as much as 6mAfrom the VL supply. To ensure VL stability, bypass the VLpin with a 1µF or greater capacitor.

The MAX1647 has an internal ±2% accurate 3.9V refer-ence voltage. An external reference can be used toincrease the charger’s accuracy. Use a 4.096V reference,such as the MAX874, for compliance with the Intel/Duracell smart-battery specification. Voltage-settingaccuracy is ±0.65%, so the total voltage accuracy is theaccuracy added to the reference accuracy. For 1% totalvoltage accuracy, use a reference with ±0.35% or greateraccuracy. If the internal reference is used, bypass it witha 0.1µF or greater capacitor.

MAX1647 Logic

The MAX1647 uses serial data to control its operation. Theserial interface complies with the SMBus specification (seeSystem Management Bus Specification, from IntelArchitecture Labs; /IAL/power-mgm.html; Intel Architecture Labs: 800-253-3696).Charger functionality complies with the Intel/DuracellSmart Charger Specification for a level 2 charger.

The MAX1647 uses the SMBus Read-Word and Write-Word protocols to communicate with the battery it ischarging, as well as with any host system that monitorsthe battery to charger communications. The MAX1647never initiates communication on the bus; it onlyreceives commands and responds to queries for statusinformation. Figure 7 shows examples of the SMBusWrite-Word and Read-Word protocols.

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Figure 7. Write-Word and Read-Word Examples

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