LTC2944
14
2944f
For more information www.linear.com/LTC2944
Alert Thresholds Registers (E,F,G,H,K,L,M,N,Q,R,S,
T,W,X)
For each of
the measured quantities (battery charge,
voltage, current and temperature) the LTC2944 features
high and low threshold registers. At power-up, the high
thresholds are set to FFFFh while the low thresholds are set
to 0000h, with the effect of disabling them. All thresholds
can be programmed to a desired value via I
2
C. As soon
as a measured quantity exceeds the high threshold or
falls below the low threshold, the LTC2944 sets the cor
-
responding flag in the status register and pulls the ALCC
pin low if alert mode is enabled via bits B[2:1].
Accumulated Charge Register (C,D)
The coulomb counting circuitry in the LTC2944 integrates
current through the sense resistor. The result of this charge
integration is stored in the 16-bit accumulated charge
register (registers C, D). As the LTC2944 does not know
the actual battery status at power-up, the accumulated
charge register (ACR) is set to mid-scale (7FFFh). If the
host knows the status of the battery, the accumulated
charge (C[7:0]D[7:0]) can be either programmed to the
correct value via I
2
C or it can be set after charging to FFFFh
(full) by pulling the ALCC pin low if charge complete mode
is enabled via bits B[2:1]. Note that before writing to the
accumulated charge registers, the analog section should
be temporarily shut down by setting B[0] to 1. In order to
avoid a change in the accumulated charge registers between
reading MSBs C[7:0] and LSBs D[7:0], it is recommended
to read them sequentially as shown in Figure 11.
Voltage Registers (I,J), and Voltage Threshold
Registers (K,L,M,N)
The result of the 16-bit ADC conversion of the voltage at
SENSE
–
is stored in the voltage registers (I, J).
From the result of the 16-bit voltage registers I[7:0]J[7:0]
the measured voltage can be calculated as:
V
SENSE
–
=70.8V •
h
FFFF
h
=70.8V •
DEC
65535
Example 1: a register value I[7:0] = B0h and J[7:0] = 1Ch
corresponds to a voltage on SENSE
–
of:
V
SENSE
–
=70.8V •
h
FFFF
=70.8V •
DEC
65535
≈ 48.705V
Example 2: To set a low level threshold for the battery
voltage of 31.2V, register M should be programmed to
70h and register N to D0h.
Current Registers (O,P), and Current Threshold
Registers (Q,R,S,T)
The result of the current conversion is stored in the cur
-
rent registers (O,P).
As the ADC resolution is 12 bits in current mode, the
lowest four bits of the combined current registers (O, P)
are always zero.
The ADC measures battery current by converting the volt
-
age, V
SENSE
, across the sense resistor R
SENSE
. Depending
whether the battery is being charged or discharged the
measured voltage drop on R
SENSE
is positive or negative.
The result is stored in registers O and P in excess –32767
representation. O[7:0] = FFh, P[7:0] = F0h corresponds to
the full scale positive voltage 64mV. While O[7:0] = 00h,
P[7:0] = 00h corresponds to the full scale negative volt
-
age –64mV. The
battery
current can be obtained from the
two byte register O[7:0]P[7:0] and the value of the chosen
sense resistor R
SENSE
:
I
BAT
=
V
SENSE
R
SENSE
=
64mV
R
SENSE
•
RESULT
h
– 7FFF
h
7FFF
h
⎛
⎝
⎜
⎞
⎠
⎟
=
64mV
R
SENSE
•
RESULT
DEC
– 32767
32767
⎛
⎝
⎜
⎞
⎠
⎟
Positive current is measured when the battery is charg-
ing and negative current is measured when the battery is
discharging.
applicaTions inForMaTion
