bq2205LY
SLUS581 − FEBRUARY 2004
FUNCTIONAL DESCRIPTION
During power up, VOUT is switched back to the main supply as VCC rises above the backup cell input voltage
sourcing VOUT. If V
PFD
< BC
P
on the bq2205 the switch to the main supply occurs at V
PFD
. CE
CONx
is held
inactive for time t
CER
after the power supply has reached V
PFD
, independent of the CE input, to allow for
processor stabilization.
Power-On Reset
The bq2205 provides a power-on reset, which pulls the RST pin low on power down and remains low on power
up for t
RST
after V
CC
passes V
PFD
. With valid battery voltage on BC
P
, RST remains valid for V
CC
= V
SS
. The
pull-up resistor on this pin should not exceed 10 kΩ if a push button reset is used.
Battery Low Warning
The bq2205 checks the battery voltage on power-up. The threshold for the battery warning comparator is V
BW
,
and a low level is sensed after power valid on each power up and latched after t
CER
time. The latched value
is presented at BW pin where a low indicates a low battery.
APPLICATION INFORMATION
PCB LAYOUT INFORMATION
It is important to pay special attention to the PCB layout. The following provides some guidelines:
D
To obtain optimal performance, the decoupling capacitor from input terminals to V
SS
should be placed as
close as possible to the bq2205, with short trace runs to both signal and V
SS
pins.
D
All low-current V
SS
connections should be kept separate from the high-current paths from the inputs
supplies. Use a single-point ground technique incorporating both the small signal ground path and the
power ground path.
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