ISL12021 Datasheet, PDF (23/24 Pages) Intersil Corporation – Real Time Clock with On Chip Temp Compensation ±5ppm

ISL12021
Example 2. Calculating a Capacitor Value for a
Given Backup Time
Referring to Figure 13 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
V_DD = 5.0V. As in Example 1, the V_BAT voltage will vary from
4.7V down to 1.8V. We will need to rearrange Equation 3 to
solve for capacitance:
C_BAT = dT*I/dV
(EQ. 8)
Using the terms described above, this equation becomes:
C_BAT = t_BACKUP*(I_BATAVG + I_LKG)/(V_BAT2 – V_BAT1)
(EQ. 9)
where
t_BACKUP = 60 days*86, 400s/day = 5.18 E6s
I_BATAVG = 4.387 E-7A (same as Example 1)
I_LKG = 0 (assumed)
V_BAT2 = 4.7V
V_BAT1 = 1.8V
Solving gives
C_BAT = 5.18 E6 * (4.387 E-7)/(2.9) = 0.784F
If the 30% tolerance is included for tolerances, then worst
case cap value would be:
C_BAT = 1.3 *.784 = 1.02F
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FN6451.0
March 30, 2007