App description
Super capacitor working time
The calculator determines the working time in the time keeping state when the super capacitor is powered according to the starting voltage and the ending voltage, the discharging current and the electric capacity of the capacitor.
among them:
The maximum V<sub>cap</sub> corresponds to the maximum value of V<sub>CC</sub>, and the related data can be consulted, namely: (V<sub>CC</sub> maximum value) - (diode voltage drop)
The minimum Vcap is the minimum operating voltage of the RTC oscillator. Refer to the relevant data sheet.
The maximum value of I<sub>BAT</sub> is the maximum current of the battery, and the relevant data can be consulted.
The typical value of I<sub>BAT</sub> is the typical battery current defined at 25 ° C and the nominal supply voltage, and the relevant data can be found.
The maximum value of V<sub>BAT</sub> is the maximum voltage of the V<sub>BAT</sub> pin, which can be consulted.
I<sub>BAT</sub> maximum and I<sub>BAT</sub> typical values are used to calculate the maximum I<sub>BAT</sub> and typical I<sub>BAT</sub> work Time (hours). When calculating the working time, assume that the I<sub>BAT</sub> current remains constant, regardless of the voltage at the battery input pin.
When calculating the operating time under linear I<sub>BAT</sub>, it is assumed that the oscillator current is proportional to the input voltage, which is expressed as a resistance characteristic. The equivalent resistance of the circuit is calculated using the maximum value of I<sub>BAT</sub> and the maximum value of V<sub>BAT</sub> provided in the data sheet, which is used to calculate the battery backup time. Specification sheet example
Recommended DC operating conditions (TA = -40°C to +85°C)
<table class="pure-table"> <tbody> <tr> <th>Parameter</th> <th>Symbol</th> <th>Condition</th> <th>Minimum value</th> <th>Typical value</th> <th>Maximum value</th> <th>Point position</th> </tr> <tr> <td>Supply voltage</td> <td>V<sub>CC</sub></td> <td> </td> <td>2.7</td> <td>3.0</td> <td>3.3</td> <td>V</td> </tr> <tr> <td>Supply voltage</td> <td>V<sub>BAT</sub></td> <td> </td> <td>1.3</td> <td>3.0</td> <td>3.7</td> <td>V</td> </tr> <tr> <td colspan="6">Table 1</td> <td> </td> </tr> </tbody> </table>
DC electrical characteristics (TA = -40°C to +85°C)
<table class="pure-table"> <tbody> <tr> <th>Parameter</th> <th>Symbol</th> <th>Condition</th> <th>Minimum value</th> <th>Typical value</th> <th>Maximum value</th> <th>Point position</th> </tr> <tr> <td>Battery current</td> <td>V<sub>BAT</sub></td> <td> </td> <td> </td> <td>600</td> <td>1000</td> <td>nA</td> </tr> <tr> <td colspan="6">Table 2</td> <td> </td> </tr> </tbody> </table>
Typical RTC trickle charger internal circuit
<img src="http://drr.ikcest.org/static/upload/09/09884c28-ebc3-11e9-a56a-00163e0618d6_m.jpg" />
Figure 1
Note: The diode selection circuit of some devices allows the selection of 1 or 2 series diodes, and some devices can select 0 or 1 diodes. Assume that each diode has a voltage drop of 0.7V.
Typical RTC circuit with supercapacitor backup power supply
<img src="http://drr.ikcest.org/static/upload/27/271edf0e-ebc3-11e9-b24a-00163e0618d6_m.jpg" />
Figure 2