ISL1219 Intersil Corporation, ISL1219 Datasheet - Page 24

• Manufacturer Part Number: ISL1219
• Description: Real Time Clock/Calendar
• Manufacturer: Intersil Corporation

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where:
Solving equation 4 for this example, I
Since there are 86,400 seconds in a day, this corresponds to
35.96 days. If the 30% tolerance is included for capacitor
and supply current tolerances, then worst case backup time
would be:
C
Example 2. Calculating a Capacitor Value for a
Given Backup Time
Referring to Figure 23 again, the capacitor value needs to be
calculated to give 2 months (60 days) of backup time, given
VDD = 5.0V. As in Example 1, the V
from 4.7V down to 1.8V. We will need to rearrange
Equation 2 to solve for capacitance:
Using the terms described above, this equation becomes:
where:
Solving gives:
If the 30% tolerance is included for tolerances, then worst
case cap value would be:
C
C
C
BAT
BAT
BAT
BAT
C
V
V
I
T
T
I
I
V
V
C
LKG
BATAVG
LKG
BACKUP
BACKUP
BAT2
BAT1
BAT2
BAT1
BAT
BAT
= 0.70 * 35.96 = 25.2 days
= 1.3 *.784 = 1.02F
= dT*I/dV
= T
= 0 (assumed minimal)
= 0 (assumed)
= 0.47F
= 5.18 E6 * (4.387 E-7)/(2.9) = 0.784F
= 4.7V
= 1.8V
= 4.7V
= 1.8V
BACKUP
= 4.387 E-7 A (same as Example 1)
= 0.47 * (2.9) / 4.38E-7 = 3.107E6 sec
= 60 days * 86,400 sec/day = 5.18 E6 sec
* (I
BATAVG
24
+ I
LKG
BAT
)/(V
BATAVG
voltage will vary
BAT2
= 4.387E-7 A
– V
BAT1
(EQ. 7)
(EQ. 6)
)
ISL1219
August 14, 2006
FN6314.1