
2.0 Power, Reset and Clocks
(Continued)
Revision 1.2
33
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P
Figure 1 shows the power state transitions:
Figure 1. Power State Transitions
2.1.3
The PC8741x requires a power supply voltage of 3.3V
±
10% for both the V
DD
and the V
SB
supplies. The device is designed
to operate with a Lithium backup battery supplying up to 3.6V. Therefore, it includes an internal current-limiting resistor on
the V
BAT
input to prevent the battery from shorting, as required by the UL regulations.
V
DD
, V
SB
and V
BAT
use a common ground return marked V
SS
.
To obtain the best performance, bear in mind the following recommendations.
Power Connection and Layout Guidelines
Ground Connection.
The following items must be connected to the ground layer (V
SS
) as close to the device as possible:
The five ground return (V
SS
) pins.
The decoupling capacitors of the Main power supply (V
DD
) pins.
The decoupling capacitors of the Standby power supply (V
SB
) pins.
The decoupling capacitor of the Backup battery (V
BAT
) pin.
Note that a low-impedance ground layer also improves noise isolation.
Decoupling Capacitors.
The following decoupling capacitors must be used in order to reduce EMI and ground bounce:
Main power supply (V
DD
): Place one capacitor of 0.1
μ
F on
each
V
DD
-V
SS
pin pair as close to the pin as possible. In
addition, place one 10
47
μ
F tantalum capacitor on the common net as close to the chip as possible.
Standby power supply (V
SB
): Place one capacitor of 0.1
μ
F on
each
V
SB
-V
SS
pin pair as close to the pin as possible. In
addition, place one 10
47
μ
F tantalum capacitor on the common net as close to the chip as possible.
Backup battery (V
BAT
): Place one capacitor of 0.1
μ
F on the V
BAT
pin as close to the pin as possible. In addition, place
one 4.7
10
μ
F ceramic capacitor on the common net as close to the chip as possible.
V
DD
, V
SB
and V
BAT
Off
Battery Fail
V
DD
and V
SB
Off, V
BAT
On
Power Fail
V
DD
Off, V
SB
On
Power Off
V
DD
and V
SB
On
Power On
V
BAT
On,
V
PP
Power-Up Reset
V
SB
On,
V
SB
Power-Up Reset
V
DD
On,
V
DD
Power-Up Reset
V
BAT
Off
V
SB
Off
V
DD
Off
V
SB
On (V
BAT
Off),
Cold Reset +
V
PP
Power-Up Reset
V
BAT
On
V
BAT
Off
V
SB
Off (V
BAT
Off)
V
BAT
On
V
BAT
Off