BQ2201SN datasheet(2/12 Pages) TI

2 / 12 page

As the supply continues to fall past VPFD, an internal

switching device forces VOUT to one of the two external

backup energy sources. CECON is held high by the VOUT

energy source.

During power-up, VOUT is switched back to the VCC sup-

ply as VCC rises above the backup cell input voltage

sourcing VOUT. The CECON output is held inactive for

time tCER (120 ms maximum) after the supply has

reached VPFD, independent of the CE input, to allow for

processor stabilization.

During power-valid operation, the CE input is fed

through to the CECON output with a propagation delay

of less than 10ns. Nonvolatility is achieved by hardware

hookup, as shown in Figure 1.

Energy Cell Inputs—BC1,BC2

Two primary backup energy source inputs are provided

on the bq2201. The BC1 and BC2 inputs accept a 3V pri-

mary battery, typically some type of lithium chemistry.

If no primary cell is to be used on either BC1 or BC2, the

unused input should be tied to VSS.

If both inputs are used, during power failure the VOUT

output is fed only by BC1 as long as it is greater than

2.5V. If the voltage at BC1 falls below 2.5V, an internal

isolation switch automatically switches VOUT from BC1

to BC2.

To prevent battery drain when there is no valid data to

retain, VOUT and CECON are internally isolated from

BC1 and BC2 by either of the following:

Initial connection of a battery to BC1 or BC2,or

Presentation of an isolation signal on CE.

A valid isolation signal requires CE low as VCC crosses

both VPFD and VSO during a power-down. See Figure 2.

Between these two points in time, CE must be brought

to the point of (0.48 to 0.52)*VCC and held for at least

700ns.

The isolation signal is invalid if CE exceeds

0.54*VCC at any point between VCC crossing VPFD and

VSO.

The appropriate battery is connected to VOUT and CECON

immediately on subsequent application and removal of VCC.

FG220101.eps

VCC

BC1

THS

VSS

VOUT

CECON

BC2

bq2201

VCC

CMOS

SRAM

From Address Decoder

Primary

Cell

Primary

Cell

Figure 1. Hardware Hookup (5% Supply Operation)

Oct. 1998 D

TD220101.eps

VCC

VPFD

VSO

0.5 VCC

700ns

Figure 2. Battery Isolation Signal

bq2201

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