Super Capacitor Calculator

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 V_CC maximum value, as noted in the data sheet, i.e., (V_CCMax) - (diode drop.)
• Min Vcap is the minimum, oscillator operating voltage of the RTC, as noted in the data sheet.
• I_BAT Max is the maximum battery current, as noted in the data sheet.
• I_BAT Typ is the typical battery current defined at both 25°C and the nominal supply voltage, as noted in the data sheet.
• V_BAT Max is the maximum voltage on the V_BAT pin, as noted in the data sheet.

I_BAT Max and I_BAT Typ values are used to calculate the I_BAT Max and I_BAT Typ Hours. These calculations assume that the I_BAT current remains constant, regardless of the voltage at the battery input pin.

The (Linear I_BAT) Hours calculation assumes that oscillator current is directly proportional to the voltage input, i.e., resistive in nature. The I_BAT Max and V_BAT 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

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