- Detail

Estimate the super capacitor backup time of the trickle charging real-time clock

charging circuit

Figure 1 shows a typical trickle charger circuit, specifying that the upper four bits of the Trickle Charger register are used to enable the Trickle Charger, and the lower four bits are used to select the diode differential voltage and current limiting resistance. In the figure below, diodes can be inserted into the charging path or not. The selectable resistance values are 250, 2K or 4K. Some devices provide different diode and resistance configurations (please refer to the data of relevant devices for details). The capacitor is connected between vbackup and ground (Figure 2)

Figure 1 Typical trickle charging circuit

Figure 2 Typical circuit

users can select diodes and resistors according to the maximum charging current required. If necessary, they can contact the capacitor manufacturer to understand the charging limits of the capacitor

calculate the charging current of the prepared sample

the maximum charging current can be calculated as follows: assume that the 3.3V system power supply is added to the VCC, the Trickle Charger has been enabled, no diode is used, and 2K resistance is selected. When the capacitor voltage is zero, the maximum charging current can be calculated:

imax = (VCC - diode differential voltage)/r2

= (3.3v-0v)/r2

(3.3v-0v)/2k

1.65ma

with the increase of vbackup voltage, the charging current gradually decreases

calculate the backup time

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 drawn from the capacitor, and the capacitance value

assuming that the RTC consumes a fixed current when working in vbackup, the backup time under the worst working conditions can be calculated by using the following formula, expressed in hours:

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

where C is the capacitance value, and the unit is farad

vbackupstart is the initial voltage, in volts. The voltage acting on the VCC removes the voltage drop of the diode and is used in the charging circuit

vbackupmin is the final voltage, in volts (the lowest operating voltage of the oscillator)

ib extrusion pressure fluctuation and extrusion temperature, use of cooling device ackupmax is the maximum vbackup current given in the data, and the unit is ampere

if: C = 0.2f, vbackupstart = 3.3V, vbackupmin = 1.3V, ibackupmax = 1000na, then:

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

if we need to know the typical backup time, we should use IBackup typical value (ibackuptyp) to replace IBackup maximum value (ibackupmax).The

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

hours = [0.2 (3.3 - 1.3)/(600E - 9)]/3600 = [0.2 (2.0) (600E - 9)]/3600 = 185.2

the above calculation assumes that IBackup is fixed, regardless of vback up voltage due to scientific and technological progress. The oscillator of Maxim RTC is much like a resistor, so the backup current decreases with the decrease of backup voltage. You can calculate the backup time that is closer to the actual situation

according to basic electronics, the capacitor voltage at any time can be determined by the following formula (the discharge circuit is shown in Figure 3):

V (T) so that we return the pointer of the hydraulic universal testing machine to zero = e (e-/rc)

Figure 3 Discharge circuit

where, is time, and the unit is seconds

e is the initial voltage, the unit is volts

v is the final voltage, the unit is volts

r is the load resistance, the unit is ohms

c is the capacitance value, and the unit is farad

after sorting out the above formula, we can get t:

-ln (v/e) (RC) = t

from the RTC data, we can find the minimum working voltage and the maximum vbackup of the oscillator.Current (ibackupmax). To estimate the load resistance, R, we divide vbackupmax in the data by ibackupmax (because the maximum current occurs at the maximum input voltage). In this example, vbackupmax is 3.7V, ibackupmax is 1000na, and the result is 3.7/1e-6 or 3.7 million. Assuming that the capacitance value is 0.2f, has been charged to 3.3V, ibackupmax is 1000na, and the minimum operating voltage of the oscillator is 1.3V, the backup time is calculated as follows:

-ln (vbackupmin/vbackupmax) [(vbackupmax/ibackupmax) c]=

-ln (1.3/3.3) (3.7 million 0.2) =

689353 seconds (191.5 hours)

changing the capacitance value C can determine the operation time when the backup capacitance is powered on

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