NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
Using C·V = I·t:
t= C ⋅ (VCbackupstart −VCbackupend )
backupIRTC
Assuming that the PCF8563 is used, that VCbackupstart = 3.3 V, that the RTC current consumption is 250 nA and a backup capacitor value of 0.47 F it is possible to estimate
the available backup time. The oscillator stops running when VDD drops to 1.0 V.
0.47 F ⋅ (3.3 V − 1.0 V )
t backup = 250 ⋅ 10 − 9 A = 4324000 s
As one day contains 86400 seconds this thus corresponds to 50 days. In order to reserve for capacitor and supply current tolerances and variations in temperature, a 30% margin should be included. This means reducing the backup time by 30% resulting in 35 days.
In this calculation example the leakage currents through diode D1 and through the super cap have been ignored. In a similar way the required capacitance value can be calculated if the required backup time is known.
13.4 Diode selection
In order to optimize possible backup time it is useful to select a low leakage diode for D1, i.e. a diode with a low reverse current. If without further consideration a common small signal diode is chosen, its reverse current may be in the order of the current consumption of the RTC when the RTC is just keeping time without being accessed. The common 1N4148 for example has a specified maximum reverse current of 25 nA at 25 °C which however can increase to 300 nA at 80 °C. Whether this is a problem depends on the application. If it is, special low leakage diodes are available which in some cases limit reverse current to a few pico ampere. In Table 9 some diodes are suggested which limit reverse current IR to a few nano ampere. These diodes are all available from NXP.
Table 9. | Some suggestions for diode D1 |
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| 1N4148 | BAS45A | BAS45AL | BAS716 | BAS116 | BAV170 |
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Package |
| leaded | leaded | SMD | SMD | SMD | SMD |
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Typ. reverse current IR at 25 °C | - | 0.2 nA | 0.2 nA | 0.2 nA | 3 pA | 3 pA | |
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Max. reverse current IR at 25 °C | 25 nA | 1 nA | 1 nA | 5 nA | 5 nA | 5 nA | |
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Price indication, relative w.r.t. | 1 | 7x | 8x | 3.5x | 3x | 3x | |
1N4148 |
| (0.02 $) |
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BAS116 is the cheapest alternative here and shows low leakage current. BAV170 is equally good and for the same price it offers two diodes in a
As stated before, usually it is not necessary to select a Schottky diode. In case some application requires ultra low voltage drop over the diode, an option is the PMEG3005EB which shows very small forward voltage drop at the expense of a higher reverse current.
UM10301_1 |
| © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 35 of 52 |