Application note 3816 DS 1302 estimates the super capacitor backup time of the real-time clock for streaming Charging

Original article address: Success Jun 28,200 5

Abstract: The maxim real-time clock (RTC) series includes several devices with a streaming charger, which can charge the second backup battery or capacitor. The battery or capacitor is used to maintain clock operation when the VCC powers down. The energy stored in the capacitor needs to maintain the clock for a certain period of time, depending on several factors. This application note discusses how to calculate the backup time based on the capacitor size.



For more information, see application note 3816: "selecting a backup source for real-time clocks ". Click the link below

Http://china.maxim-ic.com/app-notes/index.mvp/id/3816

Charging Circuit

Figure 1A typical flat charger circuit is provided, which specifies that the four-bit high of the flat charger register is used to enable the flat charger, and the four-bit low is used to select the diode pressure difference and throttling resistance. You can insert a diode in the charging path, or choose not to use a diode. The available resistance values are 250 Ω, 2 kb, or 4 kb. Some devices provide different diode and resistor configurations (For details, refer to the related device data ). The capacitor is connected between the vbackup and the ground (Figure 2).

Figure 1. Typical streaming charging circuit

Figure 2. Typical Circuits

You can select the diode and resistor based on the maximum charging current you need. Contact the capacitor manufacturer to learn about the charging limits of the capacitor.

Calculate charging current

The maximum charging current can be calculated as follows: assuming that the system power of 3.3v is added to VCC, the dropper charger is enabled, no diode is used, and a 2 k Ω resistor is selected. When the capacitor voltage is zero, the maximum charge current can be calculated:

IMAX = (VCC-diode pressure difference)/R2
= (3.3 V-0 V)/R2
≈ (3.3 V-0 V)/2 k Ω
≈. 1. 65mA

As the vbackup voltage increases, the charging current gradually decreases.

Backup Time Calculation

Next, we need to calculate the required capacitance value. Given the required backup time, we also need to know several other parameters: the starting voltage and final voltage of the capacitor, the current learned from the capacitor, And the capacitance value.

Assuming that RTC consumes Fixed Current when working in vbackup, you can use the following formula to calculate the backup time under the worst working condition, expressed in hours:

[C (vbackupstart-vbackupmin)/ibackupmax]/3600

In formula, C is the capacitance value in Farah.
Vbackupstart is the initial voltage, measured in volts. The voltage applied to VCC is used to charge the circuit by removing the voltage drop of the diode.
Vbackupmin is the final voltage, measured in volts (the minimum operating voltage of the oscillator ).
Ibackupmax is the maximum vbackup current in data, measured in amps.

If: c = 0.2f, vbackupstart = 3.3 V, vbackupmin = 1.3 V, and ibackupmax = 1000na, then:

Hour = [0.2 (3.3-1.3)/(1E-6)]/3600 = [0.2 (2.0) (1E-6)]/3600 = 111.1

To understand the typical backup time, replace ibackup (ibackuptyp) with ibackup (ibackupmax ).

Therefore, if vbackuptyp is 3.3 V (typical value) and ibackuptyp is 600na (typical value), then:

Hour = [0.2 (3.3-1.3)/(600e-9)]/3600 = [0.2 (2.0) (600e-9)]/3600 = 185.2

The above calculation assumes that the ibackup is fixed and the vbackup voltage is not considered. The maxim RTC oscillator is like a resistor, so the backup current decreases with the backup voltage decreasing. You can calculate the backup time closer to the actual situation.

According to the basic electronics, the capacitor voltage at any time can be determined by the following method (discharge circuit suchFigure 3):

V (t) = E (e-tau/RC)

Figure 3. discharge circuit

Where, Tau is the time, in seconds
E is the initial voltage, unit: volt
V is the final voltage, measured in volts.
R is the load resistance, in Ohm
C is the capacitance value, in Farah

Sort the above formula and you can understand t:

-Ln (V/E) (RC) = T

From the RTC data, we can find the minimum operating voltage of the oscillator and the maximum vbackup current (ibackupmax ). To estimate the load resistance, we use vbackupmax in the data to divide it by ibackupmax (because the maximum current occurs at the maximum input voltage ). In this example, vbackupmax is 3.7 V, ibackupmax is 1000na, and the result is 3.7/1e-6 or 3,700,000 Ω. Assume that the capacitor value is 0.2f and is charged to 3.3 V,
Ibackupmax is 1000na, and the minimum operating voltage of the oscillator is 1.3 V. The backup time is calculated as follows:

-Ln (vbackupmin/vbackupmax) [(vbackupmax/ibackupmax) × C] =
-Ln (1.3/3.3) (3,700,000 × 0.2) =
689,353 seconds (191.5 hours)

Change the capacitance value C to determine the running time of the backup capacitor.

These calculations can be completed using a calculator on the website. The super capacitor calculator can calculate the above three formulas.