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Super Capacitor Calculator
For Trickle Charger RTCs
This calculator determines timekeeping operation using a super capacitor based upon starting and ending capacitor voltages, discharge current, and capacitor size.
Where:
- Max Vcap is the VCC maximum value, as noted in the data sheet, i.e., (VCCMax) - (diode drop.)
- Min Vcap is the minimum, oscillator operating voltage of the RTC, as noted in the data sheet.
- IBAT Max is the maximum battery current, as noted in the data sheet.
- IBAT Typ is the typical battery current defined at both 25°C and the nominal supply voltage, as noted in the data sheet.
- VBAT Max is the maximum voltage on the VBAT pin, as noted in the data sheet.
IBAT Max and IBAT Typ values are used to calculate the IBAT Max and IBAT Typ Hours. These calculations assume that the IBAT current remains constant , regardless of the voltage at the battery input pin.
The (Linear IBAT) Hours calculation assumes that oscillator current is directly proportional to the voltage input, i.e., resistive in nature. The IBAT Max and VBAT Max values from the data sheet are used to calculate the equivalent resistance of the circuit, which is used in the equation to calculate the backup time.
For details on the formulas used in the calculations, refer to application note 3517 , Estimating Super Capacitor Backup Time on Trickle-Charger Real-Time Clocks.
Example Specification Tables
RECOMMENDED DC OPERATING CONDITIONS (TA= -40°C to +85°C)
| Parameter |
Symbol |
Conditions |
Min |
Typ |
Max |
Units |
| Supply Voltage |
VCC |
|
2.7 |
3.0 |
3.3 |
V |
| Battery Voltage |
VBAT |
|
1.3 |
3.0 |
3.7 |
V |
| Table 1 |
DC ELECTRICAL CHARACTERISTICS (TA= -40°C to +85°C)
| Parameter |
Symbol |
Conditions |
Min |
Typ |
Max |
Units |
| Battery Current |
IBAT |
|
|
600 |
1000 |
nA |
| Table 2 |
Typical RTC trickle-charger internal circuit diagram
Figure 1
Note: Some device diode select circuits allow 1 or 2 series diodes, others allow 0 or 1. Assume 0.7V drop per diode.
Typical RTC with SuperCap on backup supply
Figure 2
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