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DS2437
Smart Battery Monitor
FEATURESUnique 1-Wire® interface requires only one
port pin for communicationProvides unique 64-bit serial number tobattery packsEliminates thermistors by sensing battery
temperature on-chipOn-board A/D converter allows monitoring of
battery voltage for end-of-charge and end-of-discharge determinationOn-board integrated current accumulator
facilitates gas gaugingReal-time clock in binary format40-byte nonvolatile user memory available forstorage of user data such as gas gauge and
manufacturing informationOperating range -40°C to +85°CApplications include portable computers,portable/cellular phones, consumer
electronics, and handheld instrumentation
PIN ASSIGNMENT
PIN DESCRIPTIONDQ - Data In/Out
VAD - General A/D input
VSENS+ - Battery current monitor input (+)VSENS- - Battery current monitor input (-)
NC - No connect
GND - Digital Ground
AGND - Analog Ground
X2 - Connection for 32.768 kHz XTALX1 - Connection for 32.768 kHz XTAL
VDD - Power Supply (2.7V to 10.0V)
DESCRIPTIONThe DS2437 Smart Battery Monitor provides several functions that are desirable to carry in a battery
pack: a means of tagging a battery pack with a unique serial number; a direct-to-digital temperature
sensor which eliminates the need for thermistors in the battery pack; an A/D converter which measuresthe battery voltage and current; an integrated current accumulator, which keeps a running total of all
current going into and out of the battery; a real-time clock; and 40 bytes of nonvolatile EEPROM
memory for storage of important parameters such as battery capacity, capacity remaining, and indication
of battery cycling.
Information is sent to/from the DS2437 over a 1-Wire interface, so that only one wire (and ground) needs
to be connected from a central microprocessor to a DS2437. This means that battery packs need only
have three output connectors: battery power, ground, and the 1-Wire interface.
Because each DS2437 contains a unique silicon serial number, multiple DS2437s can exist on the same1-Wire bus. This allows multiple battery packs to be charged or used in the system simultaneously.
DS243716-PIN SSOP
See Mech. Drawings Section
VAD
VSENS+
AGND
VDD
GND
VSENS-
DS2437
Applications for the smart battery pack monitor include portable computers, portable/cellular telephones,
and handheld instrumentation battery packs in which it is critical to monitor real-time battery
performance. Used in conjunction with a microcontroller in the battery pack or host system, the DS2437
provides a complete smart battery pack solution that is fully chemistry-independent. The customizationfor a particular battery chemistry and capacity is realized in the code programmed into the
microcontroller and DS2437 EEPROM, and only a software revision is necessary should a designer wish
to change battery pack chemistry.
DETAILED PIN DESCRIPTION
OVERVIEWThe block diagram of Figure 1 shows the seven major components of the DS2437:
1. 64-bit lasered ROM
2. temperature sensor
3. battery voltage A/D
4. battery current A/D
5. current accumulators
6. real-time clock
7. 40-byte nonvolatile user memory
Each DS2437 contains a unique 64-bit lasered ROM serial number so that several battery packs can be
charged/monitored by the same host system. Furthermore, other Dallas products featuring the same
1-Wire bus architecture with a 64-bit ROM can reside on the same bus; refer to the Dallas "AutomaticIdentification Data Book" for the specifications of these products.
Communication to the DS2437 is via a 1-Wire port. With the 1-Wire port, the memory and control
functions will not be available until the ROM function protocol has been established. The master mustfirst provide one of four ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, or 4)
Skip ROM. These commands operate on the 64-bit lasered ROM portion of each device and can singulate
a specific device if many are present on the 1-Wire line as well as indicate to the bus master how many
DS2437
Control function commands may be issued which instruct the DS2437 to perform a temperature
measurement or battery voltage A/D conversion. The result of these measurements will be placed in the
DS2437’s memory map, and may be read by issuing a memory function command which reads the
contents of the temperature and voltage registers. Additionally, the charging/discharging battery current ismeasured without user intervention, and again, the last completed result is stored in DS2437 memory
space. The DS2437 uses these current measurements to update three current accumulators: one stores net
charge for gas gauge calculations, the second accumulates the total charging current over the life of the
battery, and the remaining accumulator tallies battery discharge current. The real time clock data, which
can be used in calculating battery self-discharge or time-related charge termination limits, also resides inthe DS2437 memory map and can be extracted with a control function command. The nonvolatile user
memory of the DS2437 consists of 40 bytes of EEPROM. These locations may be used to store any data
the user wishes and are written to using a memory function command. All data and commands are read
and written least significant bit first.
PARASITE POWERThe block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” powerwhenever the DQ pin is high. DQ will provide sufficient power as long as the specified timing and
voltage requirements are met (see the section titled “1-Wire Bus System”). The advantage of parasite
power is that the ROM may be read in absence of normal power, i.e., if the battery pack is completely
discharged.
DS2437 BLOCK DIAGRAM Figure 1
DS2437
OPERATION – MEASURING TEMPERATUREThe DS2437 measures temperatures through the use of an onboard proprietary temperature measurement
technique.
The temperature reading is provided in a 13-bit, two’s complement reading, which provides 0.03125°C ofresolution. Table 1 describes the exact relationship of output data to measured temperature. The data is
transmitted serially over the 1-Wire interface. The DS2437 can measure temperature over the range of
-55°C to +125°C in 0.03125°C increments. For Fahrenheit usage, a lookup table or conversion factor
must be used.
Note that temperature is represented in the DS2437 in terms of a 0.03125°C LSb, yielding the following
13-bit format. The 3 least significant bits of the temperature register will always be 0. The remaining 13
bits contain the two’s complement representation of the temperature in °C, with the MSb holding the sign
(S) bit. See “Memory Map” section for the TEMPERATURE REGISTER address location.
Temperature/Data Relationships Table 1
OPERATION – MEASURING BATTERY VOLTAGEThe on-board analog-to-digital converter (ADC) has 10 bits of resolution and will perform a conversion
when the DS2437 receives a command protocol (Convert V) instructing it to do so. The result of thismeasurement is placed in the 2-byte VOLTAGE REGISTER. The range for the DS2437 ADC is 0V to
10V; this range is suitable for NiCd or NiMH battery packs up to six cells and for lithium ion battery
packs of two cells. The full-scale range of the ADC is scaled to 10.23V, resulting in a resolution of 10
mV.
While the ADC has a range that extends to 0V, it is important to note that the battery voltage can also be
the supply voltage to the DS2437. As such, the accuracy of the ADC begins to degrade below battery
voltages of 2.7V, and the ability to make conversions is limited by the operating voltage range of the
DS2437.
DS2437
Voltage is expressed in this register in scaled binary format, as outlined in Table 2. Note that while codes
exist for values below 2.7V, accuracy of the ADC and the limitation on the DS2437’s supply voltage
make it unlikely that these values would be used in actual practice. See “Memory Map” section for the
VOLTAGE REGISTER address location.
Voltage/Data Relationships Table 2For applications requiring a general purpose voltage A/D converter, the DS2437 can be configured so that
the result of a Convert V command will place the scaled binary representation of the voltage on the VAD
input (as opposed to the VDD input) into the VOLTAGE REGISTER in the same format described in
Table 2. Depending upon the state of the configuration register, either (but not both) the VDD or VAD
voltage will be stored in the VOLTAGE REGISTER upon receipt of the Convert V command. Refer tothe description of the Configuration Register in the Memory Map section for details. If the VAD input is
used as the voltage input, the A/D will be accurate for 0V < VAD < 2VDD over the range 2.7V < VDD <
5.0V. Recall that the battery voltage A/D (VDD input) loses accuracy as VDD falls below 2.7V. This
feature gives the user the ability to have a voltage A/D that meets spec accuracy for inputs over the entire
range of 0V < VAD < 10V for VDD = 5.0V.
OPERATION – MEASURING BATTERY CURRENTThe DS2437 features a sigma-delta A/D converter that effectively measures the current flow into and out
of the battery pack. It does so in the background at a rate of 32 measurements/sec; thus, no command is
required to initiate current flow measurements. However, the DS2437 will only perform current A/D
measurements if the IAD bit is set to 1 in the CONFIGURATION REGISTER. The DS2437 measures
current flow in and out of the battery through the VSENS pins; the voltage from the VSENS+ pin to theVSENS- pin is considered to be the voltage across the current sense resistor, RSENS. While the VSENS+
terminal may be tied directly to the cell side of the RSENS resistor, we recommend using an RC low pass
filter between the other side of RSENS and VSENS-. Using a 47 kΩ=(max) resistor (RF) and a 0.1 μF
capacitor (CF), the filter cutoff is approximately 32 Hz. The current A/D measures at a rate of 32 times
per second, or once every 31.25 ms. This filter will capture the effect of many current spikes and will thus
