Keywords: tricklecharger,trickle,charger,supercap,super cap, app note 3517
[C(VBACKUPSTART - VBACKUPMIN)/IBACKUPMAX]/3600Where:
Hours = [0.2(3.3 - 1.3)/(1e - 6)]/3600 = [0.2(2.0)/(1e - 6)]/3600 = 111.1If we want to know what the typical backup time should be, we would substitute the IBACKUP typical value (IBACKUPTYP)for IBACKUP maximum (IBACKUPMAX).
Hours = [0.2(3.3 - 1.3)/(600e - 9)]/3600 = [0.2(2.0)(600e - 9)]/3600 = 185.2These calculations assume that IBACKUP is constant, regardless of the voltage on VBACKUP. The oscillator on Maxim RTCs tends to act more like a resistor, so that backup current tends to decrease with the backup voltage. It should, therefore, be possible to calculate a more realistic backup time.
V(t) = E(e-τ/RC)
-ln(V/E)(RC) = tFrom the RTC data sheet, we can get the minimum oscillator operating voltage as well as the maximum VBACKUP current (IBACKUPMAX). To estimate the load resistance, R, we divide the data sheet VBACKUPMAX by IBACKUPMAX (because the worse-case current occurs at the maximum input voltage). For this example, VBACKUPMAX is 3.7V and IBACKUPMAX is 1000nA, or 3.7/1e-6 or 3,700,000Ω. Assuming that the capacitor value is 0.2F and has been charged to 3.3V, that the IBACKUPMAX is 1000nA, and that the minimum oscillator operating voltage is 1.3V, the backup time would be calculated as:
-ln(VBACKUPMIN/VBACKUPMAX)[(VBACKUPMAX/IBACKUPMAX) × C] =By changing the value of C, the estimated operating time while running from the backup capacitor can be determined.
-ln(1.3/3.3)(3,700,000 × 0.2) =
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APPLICATION NOTE 3517,AN3517, AN 3517, APP3517, Appnote3517, Appnote 3517
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