TCA9517 Datasheet, PDF (9/18 Pages) Texas Instruments – LEVEL-TRANSLATING I2C BUS REPEATER

TCA9517
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APPLICATION INFORMATION
SCPS242 – DECEMBER 2012
A typical application is shown in Figure 6. In this example, the system master is running on a 3.3-V I²C bus, and
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 TCA9517 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 TCA9517 is pulled low by a driver on the I²C bus, a comparator detects the falling edge
when it goes below 0.3 V_CCA 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 TCA9517 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 8 and Figure 9. If the bus master in Figure 6
were to write to the slave through the TCA9517, waveforms shown in Figure 8 would be observed on the A bus.
This looks like a normal I²C 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.
On the B-side bus of the TCA9517, the clock and data lines would have a positive offset from ground equal to
the V_OL of the TCA9517. After the eighth clock pulse, the data line is pulled to the V_OL of the slave device, which
is very close to ground in this example. At the end of the acknowledge, the level rises only to the low level set by
the driver in the TCA9517 for a short delay, while the A-bus side rises above 0.3 V_CCA and then continues high.
3.3 V
1.2 V
10 kW
SDA
BUS
MASTER
400 kHz
SCL
10 kW
10 kW
V V _CCB
CCA
SDAB SDAA
SCLB _SCLA
TCA9517
EN
10 kW
SDA
SCL
SLAVE
400 kHz
BUS B
BUS A
Figure 6. Typical Application
V_ILC AND PULL-UP RESISTOR SIZING
For the TCA9517 to function correctly, all devices on the B-side must be able to pull the B-side below the voltage
input low contention level (V_ILC). This means that the V_OL of any device on the B-side must be below 0.4V.
V_OL of a device can be adjusted by changing the I_OL through the device which is set by the pull-up resistance
value. The pull-up resistance on the B-side must be carefully selected to ensure that logic levels will be
transferred correctly to the A-side.
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