Datasheet

www.ti.com
RESET Input
Power-On Reset
Voltage Translation
2
Maximum
Typical
Minimum
V
CC
(V)
4.543.532.5 5 5.5
1
5
4.5
4
3.5
3
2.5
2
1.5
V
pass
(V)
PCA9546A
4-CHANNEL I
2
C AND SMBus SWITCH
WITH RESET FUNCTION
SCPS148E – OCTOBER 2005 – REVISED JANUARY 2008
Table 1. Control Register Write (Channel Selection), Control Register Read (Channel Status)
(1)
B7 B6 B5 B4 B3 B2 B1 B0 COMMAND
0 Channel 0 disabled
X X X X X X X
1 Channel 0 enabled
0 Channel 1 disabled
X X X X X X X
1 Channel 1 enabled
0 Channel 2 disabled
X X X X X X X
1 Channel 2 enabled
0 Channel 3 disabled
X X X X X X X
1 Channel 3 enabled
No channel selected,
0 0 0 0 0 0 0 0
power-up/reset default state
(1) Several channels can be enabled at the same time. For example, B3 =0, B2 = 1, B1 = 1, B0 = 0 means that channels 0 and 3 are
disabled, and channels 1 and 2 are enabled. Care should be taken not to exceed the maximum bus capacity.
The RESET input is an active-low signal that may be used to recover from a bus-fault condition. When this signal
is asserted low for a minimum of t
WL
, the PCA9446A resets its registers and I
2
C state machine and deselects all
channels. The RESET input must be connected to V
CC
through a pullup resistor.
When power is applied to V
CC
, an internal power-on reset holds the PCA9546A in a reset condition until V
CC
has
reached V
POR
. At this point, the reset condition is released, and the PCA9546A registers and I
2
C state machine
are initialized to their default states, all zeroes, causing all the channels to be deselected. Thereafter, V
CC
must
be lowered below 0.2 V to reset the device.
The pass-gate transistors of the PCA9546A are constructed such that the V
CC
voltage can be used to limit the
maximum voltage that will be passed from one I
2
C bus to another.
Figure 3 shows the voltage characteristics of the pass-gate transistors (note that the graph was generated using
the data specified in the electrical characteristics section of this data sheet). In order for the PCA9546A to act as
a voltage translator, the V
pass
voltage must be equal to or lower than the lowest bus voltage. For example, if the
main bus is running at 5 V, and the downstream buses are 3.3 V and 2.7 V, then V
pass
must be equal to or below
2.7 V to effectively clamp the downstream bus voltages. As shown in Figure 3 , V
pass
(max) is at 2.7 V when the
PCA9546A supply voltage is 3.5 V or lower, so the PCA9546A supply voltage could be set to 3.3 V. Pullup
resistors then can be used to bring the bus voltages to their appropriate levels (see Figure 12 ).
Figure 3. V
pass
Voltage vs V
CC
Copyright © 2005 – 2008, Texas Instruments Incorporated Submit Documentation Feedback 5
Product Folder Link(s): PCA9546A