BQ2204A Datasheet, PDF (3/12 Pages) Texas Instruments – X4 SRAM Nonvolatile Controller Unit

Energy Cell Inputs—BC₁, BC₂
Two backup energy source inputs are provided on the
bq2204A. The BC₁ and BC₂ inputs accept a 3V primary
battery (non-rechargeable), typically some type of lith-
ium chemistry. If no primary cell is to be used on either
BC₁ or BC₂, the unused input should be tied to V_SS.
VCC falling below VPFD starts the comparison of BC1
and BC2. The BC input comparison continues until VCC
rises above VSO. Power to VOUT begins with BC1 and
switches to BC2 only when VBC1 is less than VBC2 mi-
nus VBSO. The controller alternates to the higher BC
voltage only when the difference between the BC input
voltages is greater than VBSO. Alternating the backup
batteries allows one-at-a-time battery replacement and
efficient use of both backup batteries.
To prevent battery drain when there is no valid data to
retain, VOUT and CECON1-4 are internally isolated from
BC₁ and BC₂ by either of the following conditions:
s Initial connection of a battery to BC1 or BC2, or
s 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
CECON1–4 immediately on subsequent application and
removal of VCC.
bq2204A
VCC
VPFD
CE
VSO
0.5 VCC
700ns
TD220201.eps
Figure 2. Battery Isolation Signal
Truth Table
Input
Output
CE
A
B
CECON1
CECON2
CECON3
CECON4
H
X
X
H
H
H
H
L
L
L
L
H
H
H
L
H
L
H
L
H
H
L
L
H
H
H
L
H
L
H
H
H
H
H
L
Dec. 1992 B
3