NXP Semiconductors UM10301 PCF2123 - Manual (Page 39)

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 39 of 52
Fig 16. Protection diodes
.
16. Hints to keep power consumption low
For some applications low power consumption is a key requirement. Power consumption
can be minimized by applying several measures:
• Select PCF8563 (I 2
C) or PCF2123 (SPI-bus). The PCF2123 has the lowest power
consumption which is further decreased when a crystal is selected that has a load
capacitance of 7 pF instead of the more standard 12.5 pF (a lower load capacitance
results in a lower power consumption, but not all RTCs allow to select a crystal that
requires a C
L
in that range);
• Select a crystal with a low motional resistance. The higher the resistance the higher
the losses. For lowest power consumption, aim to find a crystal with R
1
< 40 k Ω;
• The timer source clock frequency influences power consumption which is highest at
4096 Hz. The difference in power consumption between the other three clock
frequencies is negligible;
• Use lowest possible V
DD
. See the application examples given in Fig 18 and Fig 19.
During normal operation a diode charges a super cap of for example 0.47 F or 1 F.
During standby the RTC is supplied by this super cap. General purpose small signal
diodes may have leakage currents in the order of magnitude of the current
consumption of these low power RTCs. In order to maximize the RTC backup time it
is advisable to select a low leakage current diode. See section 13.4;