LTC2943
12
2943fa
For more information www.linear.com/LTC2943
To use as much of the range of the accumulated charge
register as possible the prescaler factor M should be
chosen for a given battery capacity Q
BAT
and a sense
resistor R
SENSE
as:
M≥ 4096 •
BAT
16
•
SENSE
50mΩ
M can be set to 1, 4, 16, ... 4096 by programming B[5:3]
of the control register as M = 2
2•(4 • B[5] + 2 • B[4] + B[3])
.
The default value is 4096.
In the above example of a 100mAh battery and an R
SENSE
of 50mΩ, the prescaler should be programmed to
M = 64. The q
LSB
is then 5.313μAh and the battery capacity
corresponds to roughly 18821 q
LSB
s.
Figure 3 illustrates the best choice for prescaler value M
and the sense resistor as function of the ratio between
battery capacity (Q
BAT
) and maximum current (I
MAX
). It
can be seen, that for high current applications with low
battery capacity the prescaler value should be reduced,
whereas in low current applications with a large battery
the sense resistor should be reduced with respect to its
default value of 50mV/I
MAX
.
ADC Mode B[7:6]
The LTC2943 features an ADC which measures either
voltage on SENSE
–
(battery voltage), voltage difference
between SENSE
+
and SENSE
–
(battery current) or tem-
perature via an internal temperature sensor. The reference
voltage and clock for the ADC are generated internally.
The
ADC has four different modes of operation as shown
in Table 3. These modes are controlled by bits B[7:6] of
the control register. At power-up, bits
B[7:6] are set to
[00] and the ADC is in sleep mode.
A
single conversion of the three measured quantities
is initiated by setting the bit B[7:6] to [01]. After three
conversions (voltage, current and temperature), the ADC
resets B[7:6] to [00] and goes back to sleep.
The LTC2943 is set to scan mode by setting B[7:6] to
[10]. In scan mode the ADC converts voltage, current,
then temperature, then sleeps for approximately 10 sec
-
onds. It then reawakens automatically and repeats the
three conversions. The chip remains in scan mode until
reprogrammed by the host.
Programming B[7:6] to [11] sets the chip into automatic
mode where the ADC continuously performs voltage,
current and temperature conversions. The chip stays in
automatic mode until reprogrammed by the host.
Programming B[7:6] to [00] puts the ADC to sleep. If
control bits B[7:6] change within a conversion, the ADC
will complete the running cycle of conversions before
entering the newly selected mode.
A conversion of voltage requires 33ms (typical), and cur
-
rent and temperature conversions are completed in 4.5ms
(typical). At the end of each conversion, the corresponding
registers are updated. If the converted quantity exceeds
the values programmed in the threshold registers, a flag
is set in
the status register and the ALCC
pin is pulled low
(if alert mode is enabled).
During ADC conversions additional currents are sunk from
SENSE
+
and SENSE
–
, refer to the Electrical Characteristics
table for details.
APPLICATIONS INFORMATION
Figure 3. Choice of Sense Resistor and Prescaler as Function of Battery Capacity and Maximum Current
2943 F03
M = 1
0.005h Q
BAT
/I
MAX
0.08h 0.34h 1.4h 5.5h 22h
M = 4 M = 16 M = 64 M = 256 M = 1024 M = 4096
0.02h
R
SENSE
≤
50mV
I
MAX
R
SENSE
≤
0.34mAh • 2
16
Q
BAT
• 50mΩ
