PCF2129AT datasheet(14/68 Pages) NXP

14 / 68 page

PCF2129A_1

Product data sheet

Rev. 01 — 13 January 2010

14 of 68

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

8.5 Power management functions

The PCF2129A has two power supply pins and one power output pin:

• VDD - the main power supply input pin

• VBAT - the battery backup input pin

• BBS - battery backed output voltage pin (equal to the internal power supply)

The PCF2129A has two power management functions implemented:

• Battery switch-over function

• Battery low detection function

The power management functions are controlled by the control bits PWRMNG[2:0] in

[1]

Default value.

[2]

When the battery switch-over function is disabled, the PCF2129A works only with the power supply VDD;

VBAT must be put to ground and the battery low detection function is disabled.

8.5.1 Battery switch-over function

The PCF2129A has a backup battery switch-over circuit which monitors the main power

supply VDD and automatically switches to the backup battery when a power failure

condition is detected.

One of two operation modes can be selected:

• Standard mode: the power failure condition happens when:

VDD < VBAT AND VDD <Vth(sw)bat

• Direct switching mode: the power failure condition happens when VDD < VBAT.

Direct switching from VDD to VBAT without requiring VDD to drop below Vth(sw)bat

Vth(sw)bat is the battery switch threshold voltage. Typical value is 2.5 V.

Table 13.

Power management control bit description

PWRMNG[2:0]

Function

000

[1]

battery switch-over function is enabled in standard mode;

battery low detection function is enabled

001

battery switch-over function is enabled in standard mode;