PCA9508 Datasheet, PDF (6/21 Pages) NXP Semiconductors – Hot swappable level translating I2C-bus repeater

NXP Semiconductors
PCA9508
Hot swappable level translating I²C-bus repeater
6.5 I²C-bus systems
As with the standard I²C-bus system, pull-up resistors are required to provide the logic
HIGH levels on the buffered bus (standard open-collector configuration of the I²C-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 I²C-bus devices in addition to SMBus devices. Standard-mode I²C-bus devices
only specify 3 mA output drive; this limits the termination current to 3 mA in a generic
I²C-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, I²C/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 I²C-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.
3.3 V
1.2 V
10 kΩ
SDA
SCL
BUS
MASTER
400 kHz
10 kΩ
V_CC(B)
SDAB
SCLB
10 kΩ
V_CC(A)
SDAA
SCLA
PCA9508
EN
10 kΩ
SDA
SCL
SLAVE
400 kHz
bus B
Fig 4. Typical application
bus A
002aac654
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 I²C-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 I²C-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.
PCA9508_1
Product data sheet
Rev. 01 — 28 April 2008
© NXP B.V. 2008. All rights reserved.
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