With the addition of the DS2770 and DS2720 to the Dallas lithium battery packSemiconductor product line, it is now possible to design an affordable cell pack that supports charge control, power control, fuel gauging, cell protection, time keeping, and pack identification. The circuit shown in Figure 1 is designed to accomplish all this by replacing an existing protection or protection/charge control circuit in a cell pack.
Figure 1. Cell Pack Circuit Diagram.
The Circuit
In Figure 1, the DS2720 is a protector for the cell, the DS2415 is a real-time clock (RTC), and the lithium battery packDS2770 is a charge controller/fuel gauge. All three devices share a common ground, supply, and communication line. All external capacitors and resistors shown are for signal filtering and ESD lithium battery packprotection. The host device is powered from the PACK+ and PACK- pins on the left-hand side of the circuit. Communication to the pack takes place over the standard Dallas 1-Wire™ interface labeled as DATA. The optional PS connection is an active low pack enable input designed to be connected to the system's ON/OFF switch. The CHARGE SOURCE pin can handle an input of up to 15V that is current limited to the charge rating of the cell. The entire circuit draws less than 100µA typical when in active mode and less than 3µA typical when the pack is disconnected.Protection Features
The DS2720 IC, lithium battery pack located in the middle of Figure 1, provides all the safety features required for a single Li- Ion or Lithium Polymer cell circuit through the use of external N-FETs. The cell is protected from over charging, excessive depletion, high discharge currents, and high temperatures. The lithium battery packDS2720 has a trickle charge feature allowing a severely depleted cell to be recovered by using the standard system charger. The host software is able to determine what condition caused the fault and report it.Note that the protection FETs are mounted on the high side of the circuit between the host/charge source and the positive terminal of the lithium battery pack battery as shown in Figure lithium battery pack1. It is preferable to power the DS2770 and DS2415 directly from the battery. This insures that their data will be maintained during a protection fault or when the pack is in sleep mode when the FETs are disabled and data would otherwise be lost. It is possible to do this without violating protection rules because high-side FETs block any possible secondary charge path through lithium battery pack the communication lines that would be a concern with low-side FETs.