PCF2127ATSLASH1 datasheet(22/80 Pages) NXP

22 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

22 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.6.4 Extra power fail detection function

The PCF2127A 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 VPFI < 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 VDD (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 VDD falls below a desired threshold voltage, Vth(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 (= VBBS)

VBAT

BLF

Vth(bat)low

(= 2.5 V)

VBAT

VDD = VBBS

INT

Fig 9.

Typical application of the extra power fail detection function

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PFO

PFI

RPU

VDD

VSS

PCF2127A

1.25 V

(internal)

PCF2127AT/1 Datasheet(PDF) 22 Page - NXP Semiconductors