Datasheet LTC4150 (Analog Devices) - 8

LTC4150
APPLICATIONS INFORMATION SENSE VOLTAGE INPUT AND FILTERS Coulomb Counting
Since the overall integration time is set by internally trim- The LTC4150’s transfer function is quantifi ed as a volt- ming the LTC4150, no external timing capacitor or trimming age to frequency gain GVF, where output frequency is the is necessary. The only external component that affects number of interrupts per second and input voltage is the the transfer function of interrupts per coulomb of charge differential drive VSENSE across SENSE+ and SENSE–. The is the sense resistor, RSENSE. The common mode range number of interrupts per second will be: for the SENSE+ and SENSE– pins is VDD ±60mV, with a f = G maximum differential voltage range of ±50mV. SENSE+ is VF • ⏐VSENSE⏐ (2) normally tied to V where DD, so there is no common mode issue when SENSE– operates within the 50mV differential limit VSENSE = IBATTERY • RSENSE (3) relative to SENSE+. Therefore, Choose RSENSE to provide 50mV drop at maximum charge or discharge current, whichever is greater. Calculate f = GVF • ⏐IBATTERY • RSENSE⏐ (4) RSENSE from: Since I • t = Q, coulombs of battery charge per INT pulse mV 50 can be derived from Equation 4: RSENSE = (1) IMAX 1 One INT = Coulombs (5) G • R The sense input range is small (±50mV) to minimize the VF SENSE loss across RSENSE. To preserve accuracy, use Kelvin Battery capacity is most often expressed in ampere- connections at RSENSE. hours. The external fi lter capacitor, CF, operates against a total 1Ah = 3600 Coulombs (6) on-chip resistance of 4k to form a lowpass fi lter that averages battery current and improves accuracy in the Combining Equations 5 and 6: presence of noise, spikes and ripple. 4.7μF is recom- 1 mended for general applications but can be extended to One INT = [Ah] (7) 3600 • G • R higher values as long as the capacitor’s leakage is low. VF SENSE A 10nA leakage is roughly equivalent to the input offset or error of the integrator. Ceramic capacitors are suitable for this use. 1Ah = 3600 • GVF • RSENSE Interrupts (8) Switching regulators are a particular concern because The charge measurement may be further scaled within they generate high levels of current ripple which may fl ow the microcontroller. However, the number of interrupts, through the battery. The V coulombs or Ah all represent battery charge. DD and SENSE+ connection to the charger and load should be bypassed by at least 4.7μF The LTC4150’s transfer function is set only by the value at the LTC4150 if a switching regulator is present. of the sense resistor and the gain GVF. Once RSENSE is The LTC4150 maintains high accuracy even when Burst selected using Equation 1, the charge per interrupt can Mode® switching regulators are used. Burst pulse “on” be determined from Equation 5 or 7. levels must be within the specifi ed differential input volt- Note that RSENSE is not chosen to set the relationship age range of 50mV as measured at C + – F and CF . To retain between ampere-hours of battery charge and number of accurate charge information, the LTC4150 must remain interrupts issued by the LTC4150. Rather, RSENSE is chosen enabled during Burst Mode operation. If the LTC4150 to keep the maximum sense voltage equal to or less than shuts down or VDD drops below 2.5V, the part resets and the LTC4150’s 50mV full-scale sense input. charge information is lost. 4150fc 8 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