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LTC1325

TI I G DIAGRA

START

HI-Z

BATP

MSBF

NULL

CLK

DIN

DOUT

COMMAND WORD

MSB-FIRST DATA (MSBF = 1)

ADC DATA

STATUS WORD

START

HI-Z

LTC1325 • TD

HI-Z

BATP

VR1

NULL

CLK

DIN

DOUT

COMMAND WORD

NOTE: THE TIMING DIAGRAM SHOWS TWO POSSIBLE COMMAND WORDS.

REFER TO FUNCTIONAL DESCRIPTION FOR INFORMATION ON HOW TO

CONSTRUCT THE COMMAND WORD

ADC DATA

STATUS WORD

MSB-FIRST DATA (MSBF = 0)

GENERAL DESCRIPTION

During normal operation, a command word is shifted into

the chip via the serial interface, then an ADC measurement

is made and the 10-bit reading and chip status word are

shifted out. The command word configures the LTC1325

and forces it into one of five modes: power shutdown, idle,

discharge, charge or gas gauge mode.

In the power shutdown mode, the analog section is turned

off and the supply current drops to 30

µA. The voltage

regulator, which provides power to the internal analog

circuitry and external bias networks, is shut down. The

voltage divider across the battery is disconnected and only

the voltage regulator for the serial interface logic is left on.

During the idle mode, the chip is fully powered but the

discharge, charge, and gas gauge circuits are off. The chip

may be placed in the idle mode momentarily while charg-

ing the battery, allowing an ADC measurement to be made

without any switching noise from the PWM current source

affecting the accuracy of the reading. The mode command

bits are picked off as they appear at DIN, allowing the

charging loop to turn off and settle while the remainder of

the command word is being shifted in.

FUNCTIONAL DESCRIPTIO

During the discharge mode, the battery is discharged by

an external transistor and series resistor. The battery is

monitored for fault conditions.

In the charge mode, the

µP monitors the battery’s voltage,

temperature and ambient temperature via the 10-bit ADC.

Termination methods such as –

∆VBAT, ∆VBAT/∆Time,

∆TBAT, ∆TBAT/∆Time, ∆(TBAT – TA), maximum tempera-

ture, maximum voltage and maximum charge time may be

accurately implemented in software. The LTC1325 also

monitors the battery for fault conditions.

In the gas gauge mode, the average voltage across the

sense resistor can be measured to determine the average

battery load current. The sense voltage is filtered by an RC

circuit, multiplied by an inverting gain of four, then con-

verted by the ADC. The

µP can then accumulate the ADC

measurements and do a time average to determine the

total charge leaving the battery. The RC circuit consists of

an internal 1k resistor RF and an external capacitor CF

connected to the Filter pin.

LTC1325CSW Datasheet(PDF) 9 Page - Linear Technology

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