PCF2129AT datasheet(16/68 Pages) NXP

16 / 68 page

PCF2129A_1

Product data sheet

Rev. 01 — 13 January 2010

16 of 68

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

The direct switching mode is useful in systems where VDD is higher than VBAT at all times.

The direct switching mode is not recommended if the VDD and VBAT values are similar

(e.g. VDD = 3.3 V, VBAT ≥ 3.0 V). In direct switching mode the power consumption is

reduced compared to the standard mode because the monitoring of VDD and Vth(sw)bat is

not performed.

8.5.1.3

Battery switch-over disabled: only one power supply (VDD)

When the battery switch-over function is disabled:

• The power supply is applied on the VDD pin

• The VBAT pin must be connected to ground

• The internal power supply, available at the output pin BBS, is equal to VDD

• The battery flag (BF) is always logic 0

8.5.1.4

Battery switch-over architecture

The architecture of the battery switch-over circuit is shown in Figure 6.

Fig 5.

Battery switch-over behavior in direct switching mode with bit BIE logic 1

(enabled)

001aaj312

internal power supply (= VBBS)

cleared via interface

backup battery operation

Vth(sw)bat

(= 2.5 V)

VDD (= 0 V)

VBAT

VDD

VBBS

INT

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