Datasheet

PCA9508_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 28 April 2008 6 of 21
NXP Semiconductors
PCA9508
Hot swappable level translating I
2
C-bus repeater
6.5 I
2
C-bus systems
As with the standard I
2
C-bus system, pull-up resistors are required to provide the logic
HIGH levels on the buffered bus (standard open-collector configuration of the I
2
C-bus).
The size of these pull-up resistors depends on the system, but each side of the repeater
must have a pull-up resistor. This part designed to work with Standard-mode and
Fast-mode I
2
C-bus devices in addition to SMBus devices. Standard-mode I
2
C-bus devices
only specify 3 mA output drive; this limits the termination current to 3 mA in a generic
I
2
C-bus system where Standard-mode devices and multiple masters are possible. Under
certain conditions higher termination currents can be used.
Please see application note AN255, I
2
C/SMBus Repeaters, Hubs and Expanders for
additional information on sizing resistors and precautions when using more than one
PCA9508 in a system or using the PCA9508 in conjunction with other bus buffers.
7. Application design-in information
A typical application is shown in Figure 4. In this example, the system master is running
on a 3.3 V I
2
C-bus while the slave is connected to a 1.2 V bus. Both buses run at 400 kHz.
Master devices can be placed on either bus.
The PCA9508 is 5 V tolerant, so it does not require any additional circuitry to translate
between 0.9 V to 5.5 V bus voltages and 2.7 V to 5.5 V bus voltages.
When the A side of the PCA9508 is pulled LOW by a driver on the I
2
C-bus, a comparator
detects the falling edge when it goes below 0.5V
CC(A)
and causes the internal driver on the
B side to turn on, causing the B side to pull down to about 0.5 V. When the B side of the
PCA9508 falls, first a CMOS hysteresis type input detects the falling edge and causes the
internal driver on the A side to turn on and pull the A side pin down to ground. In order to
illustrate what would be seen in a typical application, refer to Figure 7 and Figure 8. If the
bus master in Figure 4 were to write to the slave through the PCA9508, waveforms shown
in Figure 7 would be observed on the A bus. This looks like a normal I
2
C-bus transmission
except that the HIGH level may be as low as 0.9 V, and the turn on and turn off of the
acknowledge signals are slightly delayed.
Fig 4. Typical application
002aac654
V
CC(A)
V
CC(B)
PCA9508
SDAB SDAA
SCLB SCLA
EN
10 kΩ 10 kΩ
SDA
SCL
BUS
MASTER
400 kHz
SLAVE
400 kHz
SDA
SCL
bus B bus A
1.2 V
3.3 V
10 kΩ 10 kΩ