PCF2127ATSLASH1 datasheet(20/80 Pages) NXP

20 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

20 of 80

NXP Semiconductors

PCF2127A

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.

8.6.1.3

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

8.6.1.4

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 VDD or VBAT. It has to be

assured that there are decoupling capacitors on the pins VDD, VBAT, and BBS.

8.6.2 Battery backup supply

The VBBS 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 VBBS is driven from VDD.

Fig 6.

Battery switch-over circuit, simplified block diagram

001aag061

VCC

Vth(sw)bat

VDD

VBBS

(internal

power supply)

VDD

VDD(int)

VBAT

LOGIC

comparators

logic

switches

VCC

Vth(sw)bat

VBAT

Table 18.

Output pin BBS

Battery switch-over function mode

Conditions

VBBS equals

standard

VDD > VBAT or VDD > Vth(sw)bat

VDD

VDD < VBAT and VDD < Vth(sw)bat

VBAT

direct switching

VDD > VBAT

VDD

VDD < VBAT

VBAT

disabled

only VDD available,

VBAT must be put to ground

VDD

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