Datasheet

9
LTC1325
TI I G DIAGRA
UW W
START
HI-ZHI-Z
FSBATP
D1D9 D0
MSBF
NULL
CS
CLK
D
IN
D
OUT
COMMAND WORD
MSB-FIRST DATA (MSBF = 1)
ADC DATA STATUS WORD
START
HI-Z
LTC1325 • TD
HI-Z
FSBATP
D1 D1
D9 D9
D0
VR1
NULL
CS
CLK
D
IN
D
OUT
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 30µA. 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 D
IN
, allowing the
charging loop to turn off and settle while the remainder of
the command word is being shifted in.
FUNCTIONAL DESCRIPTIO
UU
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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 –∆V
BAT
, ∆V
BAT
/∆Time,
∆T
BAT
, ∆T
BAT
/∆Time, ∆(T
BAT
– T
A
), 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 R
F
and an external capacitor C
F
connected to the Filter pin.