Datasheet LTC4150 (Analog Devices) - 7

LTC4150
OPERATION
Charge is the time integral of current. The LTC4150 mea-
CHARGE COUNTING
sures battery current by monitoring the voltage developed First, the current measurement is fi ltered by capacitor C across a sense resistor and then integrates this information F connected across pins C + and C –. This averages fast in several stages to infer charge. The Block Diagram shows F F changes in current arising from ripple, noise and spikes the stages described below. As each unit of charge passes in the load or charging current. into or out of the battery, the LTC4150 INT pin interrupts an external microcontroller and the POL pin reports the Second, the fi lter’s output is applied to an integrator with polarity of the charge unit. The external microcontroller the amplifi er and 100pF capacitor at its core. When the then resets INT with the CLR input in preparation for the integrator output ramps to REFHI or REFLO levels, switches next interrupt issued by the LTC4150. The value of each S1 and S2 reverse the ramp direction. By observing the charge unit is determined by the sense resistor value and condition of S1 and S2 and the ramp direction, polarity is the sense voltage to interrupt frequency gain G determined. The integrating interval is trimmed to 600μs VF of the LTC4150. at 50mV full-scale sense voltage. Third, a counter is incremented or decremented every
Power-On and Start-Up Initialization
time the integrator changes ramp direction. The counter When power is fi rst applied to the LTC4150, all internal effectively increases integration time by a factor of 1024, circuitry is reset. After an initialization interval, the LTC4150 greatly reducing microcontroller overhead required to begins counting charge. This interval depends on VDD and service interrupts from the LTC4150. the voltage across the sense resistor but will be at least At each counter under or overfl ow, the INT output latches 5ms. It may take an additional 80ms for the LTC4150 to low, fl agging a microcontroller. Simultaneously, the POL accurately track the sense voltage. An internal undervoltage output is latched to indicate the polarity of the observed lockout circuit monitors VDD and resets all circuitry when charge. With this information, the microcontroller can VDD falls below 2.5V. total the charge over long periods of time, developing Asserting SHDN low also resets the LTC4150’s internal an accurate estimate of the battery’s condition. Once the circuitry and reduces the supply current to 1.5μA. In this interrupt is recognized, the microcontroller resets INT with condition, POL and INT outputs are high impedance. The a low going pulse on CLR and awaits the next interrupt. LTC4150 resumes counting after another initialization Alternatively, INT can drive CLR. interval. Shutdown minimizes battery drain when both the charger and load are off. 4150fc 7 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM TIMING DIAGRAMS OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS