NXP Semiconductors UM10301 PCF2123 - Manual (Page 35)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 35 of 52
Using C·V = I·t:
( )
RTC
Cbackupend rt Cbackupsta
backup
I
V V C
t
− ⋅
=
Assuming that the PCF8563 is used, that V
Cbackupstart
= 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 V
DD
drops to 1.0 V.
( )
s
A
V V F
t
backup
000 324 4
10 250
0 . 1 3 . 3 47 . 0
9
=
⋅
− ⋅
=
−
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 D
1
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 D
1
,
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 I
R
to a few nano ampere. These diodes are all available from NXP.
Table 9. Some suggestions for diode D1
1N4148 BAS45A BAS45AL BAS716 BAS116 BAV170
Package leaded leaded SMD SMD SMD SMD
Typ. reverse current I
R
at 25 °C - 0.2 nA 0.2 nA 0.2 nA 3 pA 3 pA
Max. reverse current I
R
at 25 °C 25 nA 1 nA 1 nA 5 nA 5 nA 5 nA
Price indication, relative w.r.t.
1N4148 1
(0.02 $) 7x 8x 3.5x 3x 3x
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 3-pin package, with common
cathode. By interchanging the positions of D
2
and R
1
in Fig 12 this component can be
used such that it represents both D
1
and D
2
.
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.