Abstract: This application note presents to the reader a complete Li-Ion battery pack solution supporting fuel gauging, charge control, battery protection, timekeeping, power management, and pack identification. All functionality is provided with three Dallas Semiconductor battery management components, namely the DS2770 battery monitor and charge controller, DS2720 Li-Ion battery protection IC, and DS2415 real-time clock, along with supporting circuitry. The circuit diagram is presented along with details on each separate function.
With the addition of the DS2770 and DS2720 to the Dallas Semiconductor 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.
In Figure 1, the DS2720 is a protector for the cell, the DS2415 is a real-time clock (RTC), and the
DS2770 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
protection. 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, 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 DS2720 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 battery as shown in Figure 1. 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 the communication lines that would be a concern with low-side FETs.
Charger
One function of the DS2770 is to control cell charging using a simple current limited power source. It
performs this feature independent of cell characteristics such as manufacturer or capacity. By controlling
external PNP transistors (P1 and P2 in Figure 1) the DS2770 charges Li-Ion or Lithium polymer based
packs with constant current up to a factory set limit of either 4.1V or 4.2V and then pulse charges to top
off the cell. The DS2770 provides a secondary charge termination if the cell temperature exceeds +50°C
or a user defined maximum charge time elapses. All that is needed to initiate charging is to attach a
current limited supply (up to 15V) to the Charge Source Pad (Figure 1).
Fuel Gauge
The DS2770 also operates as a highly accurate fuel gauge. Current is measured through an internal 25mΩ
sense resistor down to a resolution of 62.5µA with a dynamic range of ±2A average current (the input
filter allows current spikes much higher than 2A to be registered in the current accumulator). The DS2770
can easily track discharge current in GSM/CDMA applications, and its internal auto compensation
maintains measurement accuracy over the entire operating range of the device. Real-time measurements
of accumulated current, voltage, and temperature, combined with cell characterization data stored in the
DS2770's EEPROM, allow a system processor to run a very accurate fuel gauge algorithm while
consuming very few system resources. Also, because the DS2770 is powered directly from the battery,
coulomb count information is not lost when the battery pack is removed or if power is lost due to a
protection fault.
Real-Time Clock
The DS2415 provides an RTC accurate to 2 minutes per month for the host system. This requires a
32kHz, 6pF external crystal attached to the X1 and X2 pins. This architecture has a huge advantage over
other systems because the DS2415 is powered directly from the battery. By locating the clock on the
inside of the protection FETs, the system is guaranteed a clock that will maintain proper time even when
system power is lost eliminating the need for super cap or button cell backup in the host.
Pack Information
The DS2770 contains 40 bytes of user accessible EEPROM and the DS2720 has an additional 8 bytes.
The pack manufacturer should use this space to store relevant pack information such as cell chemistry,
assembly date, cell characterization information for fuel gauging, etc. Once written to, the EEPROM can
be permanently locked insuring data integrity even through power loss and ESD events. In addition, each
chip has a unique 64 bit serial number that can be used for pack identification.
Summary
A more detailed description of any of the Dallas Semiconductor battery management ICs, the Dallas fuel
gauging approach, and 1-Wire communication can be found at www.maxim-ic.com under the following
titles:
DS2415 Real Time Clock
DS2720 Single Cell Li-Ion Protection IC
DS2770 Battery Monitor and Charge Controller
DS1WM 1-Wire Master
AN131 Fuel Gauging with Dallas Semiconductor Devices
AN126 1-Wire Communication through Software
Figure 2. Layout—Sample 8.1mm x 35.5mm 4 Layer Board.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
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Figure 2. Layout—Sample 8.1mm x 35.5mm 4 Layer Board.
