LTC4150 Linear Technology, LTC4150 Datasheet - Page 6

• Manufacturer Part Number: LTC4150
• Description: Coulomb Counter/ Battery Gas Gauge
• Manufacturer: Linear Technology

OPERATIO
LTC4150
Charge is the time integral of current. The LTC4150
measures battery current by monitoring the voltage devel-
oped across a sense resistor and then integrates this
information in several stages to infer charge. The Block
Diagram shows the stages described below. As each unit
of charge passes into or out of the battery, the LTC4150
INT pin interrupts an external microcontroller and the POL
pin reports the polarity of the charge unit. The external
microcontroller then resets INT with the CLR input in
preparation for the next interrupt issued by the LTC4150.
The value of each charge unit is determined by the sense
resistor value and the sense voltage to interupt frequency
gain G
Power-On and Start-Up Initialization
When power is first applied to the LTC4150, all internal
circuitry is reset. After an initialization interval, the LTC4150
begins counting charge. This interval depends on V
the voltage across the sense resistor but will be at least
5ms. It may take an additional 80ms for the LTC4150 to
accurately track the sense voltage. An internal undervolt-
age lockout circuit monitors V
when V
Asserting SHDN low also resets the LTC4150’s internal
circuitry and reduces the supply current to 1.5µA. In this
condition, POL and INT outputs are high impedance. The
LTC4150 resumes counting after another initialization
interval. Shutdown minimizes battery drain when both the
charger and load are off.
6
VF
DD
of the LTC4150.
falls below 2.5V.
U
DD
and resets all circuitry
DD
and
CHARGE COUNTING
First, the current measurement is filtered by capacitor C
connected across pins C
changes in current arising from ripple, noise and spikes in
the load or charging current.
Second, the filter’s output is applied to an integrator with
the amplifier and 100pF capacitor at its core. When the
integrator output ramps to REFHI or REFLO levels, switches
S1 and S2 reverse the ramp direction. By observing the
condition of S1 and S2 and the ramp direction, polarity is
determined. The integrating interval is trimmed to 600µs
at 50mV full-scale sense voltage.
Third, a counter is incremented or decremented every time
the integrator changes ramp direction. The counter effec-
tively increases integration time by a factor of 1024,
greatly reducing microcontroller overhead required to
service interrupts from the LTC4150.
At each counter under or overflow, the INT output latches
low, flagging a microcontroller. Simultaneously, the POL
output is latched to indicate the polarity of the observed
charge. With this information, the microcontroller can
total the charge over long periods of time, developing an
accurate estimate of the battery’s condition. Once the
interrupt is recognized, the microcontroller resets INT
with a low going pulse on CLR and awaits the next
interrupt. Alternatively, INT can drive CLR.
F
+
and C
F
–
. This averages fast
4150fa
F