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LTC1325CN

Microprocessor-controlled battery management system

Linear Technology 

LTC1325

FUNCTIONAL DESCRIPTIO

Gas Gauge Mode

Command: MOD1 = 1, MOD0 = 1, PS = 0

Status:

BATP = X, BATR = X, FMCV = X, FEDV = X,

FHTF = X, FLTF = X, tOUT = X

In the gas gauge mode, the average voltage across the

sense resistor can be measured to determine the average

battery load current. The output of the DAC is set to ground

and switches S1, S3 and S4 are closed. A1 is configured

as an inverting amplifier with R1 and R2 setting the gain

to – 4. The voltage across the sense resistor is filtered by

an RC circuit (RF, CF) amplified by A1, then converted by

the ADC.

The microprocessor can then accumulate the ADC mea-

surements and do a time average to determine the total

charge leaving the battery. The Sense pin voltage should

not be more negative than – 450mV to ensure linearity.

The RFCF circuit consists of an internal 1k resistor and an

external capacitor connected to the Filter pin. RFCF should

be longer than the measurement interval. With the serial

clock running at 100kHz, it take 380µs to shift in the

command word and shift out the ADC measurement and

status word.

Trickle Resistor

An external trickle resistor has several functions. First, it

provides a continuous trickle charge current for topping

off the battery and countering the effects of self-discharge.

Second, it can be used to condition a deeply discharged

battery for charging. The LTC1325 will not charge a battery

unless its cell voltage is above 100mV (BATR). Finally, the

resistor is required by the battery detect circuit to pull the

VBAT pin high when the battery is removed.

SERIAL INTERFACE

The LTC1325 communicates with microprocessors and

other external circuitry via a synchronous, half duplex,

4-wire serial interface. The clock CLK synchronizes the

data transfer with each bit being transmitted on the falling

edge and captured on the rising CLK edge in both transmit-

ting and receiving systems. The LTC1325 first receives

input data and then transmits back the A/D conversion

result and status word (half duplex). Because of the half

duplex operation, DIN and DOUT may be tied together

allowing transmission over just three wires: CS, CLK and

DATA (DIN/DOUT).

Data transfer is initiated by a falling chip select CS signal.

After CS falls, the LTC1325 looks for a start bit on DIN. The

start bit is the first “logical one” clocked into the DIN input

after CS goes low. The LTC1325 will ignore all leading

zeros which precede this logical one. After the start bit is

received, the 21 other control bits are shifted into the DIN

pin to configure the LTC1325 and start a conversion. After

the last command bit, the DOUT pin remains in three-state

for one clock period before it is taken low for one null bit.

Following the null bit, the conversion results and the 8

status bits are shifted out on the DOUT pin. At the end of the

data exchange, CS should be brought high.

MSB-First/LSB-First (MSBF Control Bit)

The output data of the LTC1325 is programmed for MSB-

first or LSB-first sequence using the MSFB control bit.

When MSBF = 1, data will appear on DOUT in MSB-first

format. This is followed by the 8 status bits. Logical zeros

will be filled in indefinitely following the last data bit to

accommodate longer word lengths required by some

microprocessors. When MSBF = 0, LSB-first data will

follow the MSB-first data. Regardless of the state of

MSBF, the status bits are always shifted out in the same

order (see Figure 2).

Accommodating Microprocessors with Different Word

Lengths

The LTC1325 will fill zeros indefinitely after the transmit-

ted data until CS is brought high. At that time DOUT is

disabled (three-stated). This makes for easy interfacing

to MPU serial ports with different transfer increments

including 4 bits (e.g., COP400) and 8 bits (e.g., SPI and

MICROWIRE/PLUSTM). Any word length can be accom-

modated by the correct positioning of the start bit in the

input word.

Operation with DIN and DOUT Tied Together

The LTC1325 can be operated with DIN and DOUT tied

together. This eliminates one of the lines required to

MICROWIRE/PLUS is a trademark of National Semiconductor Corp.

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