LTC2944
14
2944fa
For more information
www.linear.com/LTC2944
Example 1: a register value of O[7:0] = A8h P[7:0] = 40h
together with a sense resistor R
SENSE
= 50mΩ corresponds
to a battery current:
I
BAT
= 64mV
50mΩ
•
A840
h
– 7FFF
h
7FFF
h
=
64mV
50mΩ
•
43072 – 32767
32767
≈ 402.5mA
The positive current result indicates that the battery is
being charged.
The values in the threshold register for the current mode
Q,R,S,T are also expressed in excess –32767 representa-
tion in the same manner as the current conversion result.
The alert after a current measurement is set if the result
is higher than the value stored in the high threshold reg-
isters Q,R or lower than the value stored in the low value
registers S,T.
Example 2: In an application, the user wants to get an
alert if the absolute current through the sense resistor,
R
SENSE
, of 50mΩ exceeds 1A. This is achieved by setting
the upper threshold I
HIGH
in register [Q,R] to 1A and the
lower threshold I
LOW
in register [S,T] to –1A. The formula
for I
BAT
leads to:
I
HIGH(DEC)
= 1A
•
50mΩ
64mV
•
32767
+ 32767= 58366
I
LOW(DEC)
= –1A
•
50mΩ
64mV
•
32767
+ 32767= 7168
Leading the user to set Q[7:0] = E3h, R[7:0] = FEh for the
high threshold and S[7:0] = 1Bh and T[7:0] = FFh for the
low threshold.
Temperature Registers (U,V) and Temperature
Threshold Registers (W,X)
As the ADC resolution is 11 bits in temperature mode, the
lowest five bits of the combined temperature registers
(U, V) are always zero.
The actual temperature can be obtained from the two byte
register U[7:0]V[7:0] by:
T = 510K
•
RESULT
h
FFFF
h
= 510K
•
RESULT
DEC
65535
Example: a register value of U[7:0] = 96h V[7:0] = 96h
corresponds to ~300K or ~27°C
A high temperature limit of 60°C is programmed by setting
register W to A7h. Note that the temperature threshold
register is single byte register and only the eight MSBs of
the 11 bits temperature result are checked.
Effect of Differential Offset Voltage on Total Charge
Error
In battery gas gauges, an important parameter is the
differential offset (V
OS
) of the circuitry monitoring the
battery charge. Many coulomb counter devices perform
an analog to digital conversion of V
SENSE
, where V
SENSE
is the voltage drop across the sense resistor, and ac-
cumulate the conversion results to infer charge. In such
an architecture, the differential offset V
OS
causes relative
charge error of V
OS
/V
SENSE
. For small V
SENSE
values V
OS
can be the main source of error.
The LTC2944 performs the tracking of the charge with an
analog integrator. This approach allows to continuously
monitor the battery charge and significantly lowers the
error due to differential offset. The relative charge error
due to offset (CE
OV
) can be expressed by:
CE
OV
=
V
OS
V
SENSE
2
As example, at a 1mV input signal a differential voltage
offset V
OS
= 20µV results in a 2% error using digital
integration, whereas the error is only 0.04% (a factor of
50 times smaller!) using the analog integration approach
of LTC2944.
The reduction of the impact of the offset in LTC2944 can
be explained by its integration scheme depicted in Figure 2.
While positive offset accelerates the up ramping of the
integrator output from REFLO to REFHI, it slows the down
ramping from REFHI to REFLO thus the effect is largely
canceled as depicted in Figure 4.
applicaTions inForMaTion
