Super Capacitor Calculator

For Trickle Charger RTCs
Operating time from SuperCap
  Max Vcap   Min Vcap   Capacitor Size Vbat Max
  V   V  

Farads

V
  IBATMax   IBATTyp   Linear IBAT  
 

nA

 

nA

 

ohms (calculated)

  Hours   Hours  

Hours

 
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
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
Figure 2