BQ2050_15 Datasheet, PDF (9/27 Pages) Texas Instruments – Lithium Ion Power Gauge™ IC

Not Recommended For New Designs
bq2050
Table 6. bq2050 Current-Sensing Errors
Symbol
Parameter
INL
Integrated non-linearity
error
INR
Integrated non-
repeatability error
Typical
±2
±1
Maximum
±4
±2
Units
%
%
Notes
Add 0.1% per °C above or below 25°C
and 1% per volt above or below 4.25V.
Measurement repeatability given
similar operating conditions.
Error Summary
Capacity Inaccurate
The LMD is susceptible to error on initialization or if no
updates occur. On initialization, the LMD value in-
cludes the error between the programmed full capacity
and the actual capacity. This error is present until a
valid discharge occurs and LMD is updated (see the
DCR description on page 7). The other cause of LMD er-
ror is battery wear-out. As the battery ages, the meas-
ured capacity must be adjusted to account for changes in
actual battery capacity.
A Capacity Inaccurate counter (CPI) is maintained and
incremented each time a valid charge occurs (qualified
by NAC; see the CPI register description) and is reset
whenever LMD is updated from the DCR. The counter
does not wrap around but stops counting at 255. The ca-
pacity inaccurate flag (CI) is set if LMD has not been up-
dated following 64 valid charges.
Current-Sensing Error
Table 5 illustrates the current-sensing error as a func-
tion of VSRO. A digital filter eliminates charge and dis-
charge counts to the NAC register when VSRO is between
VSRQ and VSRD.
Communicating With the bq2050
The bq2050 includes a simple single-pin (DQ plus re-
turn) serial data interface. A host processor uses the in-
terface to access various bq2050 registers. Battery char-
acteristics may be easily monitored by adding a single
contact to the battery pack. The open-drain DQ pin on
the bq2050 should be pulled up by the host system, or may
be left floating if the serial interface is not used.
The interface uses a command-based protocol, where the
host processor sends a command byte to the bq2050.
The command directs the bq2050 to either store the next
eight bits of data received to a register specified by the
command byte or output the eight bits of data specified
by the command byte.
The communication protocol is asynchronous return-to-
one. Command and data bytes consist of a stream of
eight bits that have a maximum transmission rate of
333 bits/sec. The least-significant bit of a command or
data byte is transmitted first. The protocol is simple
enough that it can be implemented by most host proces-
sors using either polled or interrupt processing. Data
input from the bq2050 may be sampled using the pulse-
width capture timers available on some microcontrol-
lers.
If a communication error occurs, e.g. tCYCB > 6ms, the
bq2050 should be sent a BREAK to reinitiate the serial
interface. A BREAK is detected when the DQ pin is
driven to a logic-low state for a time, tB or greater. The
DQ pin should then be returned to its normal ready-
high logic state for a time, tBR. The bq2050 is now ready
to receive a command from the host processor.
The return-to-one data bit frame consists of three dis-
tinct sections. The first section is used to start the
transmission by either the host or the bq2050 taking the
DQ pin to a logic-low state for a period, tSTRH,B. The
next section is the actual data transmission, where the
data should be valid by a period, tDSU, after the negative
edge used to start communication. The data should be
held for a period, tDV, to allow the host or bq2050 to
sample the data bit.
The final section is used to stop the transmission by re-
turning the DQ pin to a logic-high state by at least a pe-
riod, tSSU, after the negative edge used to start commu-
nication. The final logic-high state should be held until
a period, tSV, to allow time to ensure that the bit trans-
mission was stopped properly. The timings for data and
break communication are given in the serial communi-
cation timing specification and illustration sections.
Communication with the bq2050 is always performed with
the least-significant bit being transmitted first. Figure 3
shows an example of a communication sequence to read
the bq2050 NAC register.
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