Datasheet

PCF2127AT All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 11 July 2013 22 of 86
NXP Semiconductors
PCF2127AT
Integrated RTC, TCXO and quartz crystal
8.6.1.4 Battery switch-over architecture
The architecture of the battery switch-over circuit is shown in Figure 6
.
The internal power supply (available on pin BBS) is equal to V
DD
or V
BAT
. It has to be
assured that there are decoupling capacitors on the pins V
DD
, V
BAT
, and BBS.
8.6.2 Battery backup supply
The V
BBS
voltage on the output pin BBS is equal to the internal power supply, depending
on the selected battery switch-over function mode:
The output pin BBS can be used as a supply for external devices with battery backup
needs, such as SRAM (see Ref. 3 “
AN10857”). For this case, Figure 7 shows the typical
driving capability when V
BBS
is driven from V
DD
.
Fig 6. Battery switch-over circuit, simplified block diagram
001aag061
V
CC
V
th(sw)bat
V
DD
V
BBS
(internal
power supply)
V
DD
V
DD(int)
V
DD(int)
V
DD(int)
V
BAT
LOGIC
comparators logic
switches
V
CC
V
th(sw)bat
V
BAT
Table 19. Output pin BBS
Battery switch-over function mode Conditions V
BBS
equals
standard V
DD
> V
BAT
OR V
DD
> V
th(sw)bat
V
DD
V
DD
< V
BAT
AND V
DD
< V
th(sw)bat
V
BAT
direct switching V
DD
> V
BAT
V
DD
V
DD
< V
BAT
V
BAT
disabled only V
DD
available,
V
BAT
must be put to ground
V
DD