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 24 of 86
NXP Semiconductors
PCF2127AT
Integrated RTC, TCXO and quartz crystal
8.6.4 Extra power fail detection function
The PCF2127AT has an extra power fail detection circuit which compares the voltage at
the power fail input pin PFI to an internal reference voltage equal to 1.25 V.
If V
PFI
< 1.25 V, the power fail output PFO is driven LOW. PFO is an open-drain, active
LOW output which requires an external pull-up resistor in any application.
The extra power fail detection function is typically used as a low voltage detection for the
main power supply V
DD
(see Figure 9).
Usually R1 and R2 should be chosen such that the voltage at pin PFI
• is higher than 1.25 V at start-up
• falls below 1.25 V when V
DD
falls below a desired threshold voltage, V
th(uvp)
, defined
by Equation 1
:
Fig 8. Battery low detection behavior with bit BLIE set logic 1 (enabled)
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internal power supply (= V
BBS
)
V
BAT
BLF
V
th(bat)low
(= 2.5 V)
V
BAT
V
DD
= V
BBS
INT
Fig 9. Typical application of the extra power fail detection function
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PFO
PFI15
14
R
PU
V
DD
V
SS
R1
R2
PCF2127AT
1.25 V
(internal)