LTC2942
APPLICATIONS INFORMATION
ters, the analog section should be shut down by setting
B[5] to 1. In order to avoid a change in the accumulated
charge registers between reading MSBs C[7:5] and LSBs
D[7:5], it is recommended to read them sequentially as
shown in Figure 15.
sets the corresponding flag in the status register and
pulls the AL/CC pin low if alert mode is enabled via bits
B[2:1]. Note that the voltage and temperature threshold
registers are single byte registers and only the 8 MSBs of
the corresponding quantity are checked. To set a low level
threshold for the battery voltage of 3ꢀ, register L should
be programmed to 85h; a high temperature limit of 65°C
is programmed by setting register O to 8Eh.
Voltage and Temperature Registers (I, J),(M, N)
The result of the 14-bit ADC conversion of the voltage at
–
SENSE is stored in the voltage registers (I, J), whereas
2
I C Protocol
the temperature measurement result is stored in the tem-
perature registers (M, N). The voltage and temperature
registers are read only.
2
TheLTC2942usesanI C/SMBuscompatible2-wireopen-
drain interface supporting multiple devices and masters
on a single bus. The connected devices can only pull the
bus wires low and they never drive the bus high. The bus
wires must be externally connected to a positive supply
voltage via a current source or pull-up resistor. When the
As the ADC resolution is 14-bit in voltage mode and 15-bit
in temperature mode, the lowest two bits of the combined
voltage registers (I, J) and the lowest six bits of the
combined temperature registers (M, N) are always zero.
From the result of the 16-bit voltage registers I[7:5]J[7:5]
the measured voltage can be calculated as:
2
busisidle, bothSDAandSCLarehigh. DataontheI Cbus
can be transferred at rates of up to 155kbit/s in standard
mode and up to 455kbit/s in fast mode.
RESULTDEC
65535
RESULTh
FFFFh
2
–
VSENSE =6V •
=6V •
Each device on the I C/SMbus is recognized by a unique
address stored in that device and can operate as either a
transmitter or receiver, depending on the function of the
device. In addition to transmitters and receivers, devices
can also be classified as masters or slaves when perform-
ing data transfers. A master is the device which initiates a
data transfer on the bus and generates the clock signals
to permit that transfer. At the same time any device ad-
dressed is considered a slave. The LTC2942 always acts
as a slave.
Example: a register value of I[7:5] = B5 and J[7:5] = 1C
h
h
–
corresponds to a voltage on SENSE of:
45084DEC
65535
B01C
h =6V •
FFFFh
–
VSENSE =6V •
≈ 4.1276V
The actual temperature can be obtained from the two byte
register C[7:5]D[7:5] by:
Figure 3 shows an overview of the data transmission for
RESULTDEC
65535
RESULTh
FFFFh
T = 600K •
= 600K •
2
fast and standard mode on the I C bus.
Start and Stop Conditions
Example: a register value of C[7:5] = 85 D[7:5] = 55
h
h
corresponds to 355K or 27°C.
When the bus is idle, both SCL and SDA must be high. A
bus master signals the beginning of a transmission with
a START condition by transitioning SDA from high to low
while SCL is high. When the master has finished com-
municating with the slave, it issues a STOP condition by
transitioning SDA from low to high while SCL is high. The
bus is then free for another transmission. When the bus is
in use, it stays busy if a repeated START (Sr) is generated
instead of a STOP condition. The repeated START (Sr)
conditions are functionally identical to the START (S).
2942f
Threshold Registers (E, F, G, H, K, L, O, P)
For each of the measured quantities (battery charge, volt-
age and temperature) the LTC2942 features a high and a
low threshold registers. At power-up, the high thresholds
are set to FFFFh while the low thresholds are set to 5555h.
All thresholds can be programmed to a desired value via
2
I C. As soon as a measured quantity exceeds the high
threshold or falls below the low threshold, the LTC2942
12