The TCA9517 is a
bidirectional buffer with level shifting capabilities for I2C and SMBus
systems. It provides bidirectional voltage-level translation (up-translation/down-translation)
between low voltages (down to 0.9 V) and higher voltages (2.7 V to 5.5 V) in mixed-mode
applications. This device enables I2C and SMBus systems to be extended
without degradation of performance, even during level shifting.
The TCA9517 buffers
both the serial data (SDA) and the serial clock (SCL) signals on the I2C
bus, thus allowing two buses of up to 400-pF bus capacitance to be connected in an
The TCA9517 has two
types of drivers: A-side drivers and B-side drivers. All inputs and I/Os are over-voltage tolerant
to 5.5 V, even when the device is unpowered (VCCB and/or
VCCA = 0 V).
The type of buffer design on the B-side
prevents it from being used in series with devices which use static voltage offset. This is because
these devices do not recognize buffered low signals as a valid low and do not propagate it as a
buffered low again.
The B-side drivers operate from 2.7 V to 5.5 V. The output low level for this internal
buffer is approximately 0.5 V, but the input voltage must be 70 mV or more below the output low
level when the output internally is driven low. The higher-voltage low signal is called a buffered
low. When the B-side I/O is driven low internally, the low is not recognized as a low by the input.
This feature prevents a lockup condition from occurring when the input low condition is released.
The A-side drivers operate from 0.9 V to 5.5 V and drive more current. They do not
require the buffered low feature (or the static offset voltage). This means that a low signal on
the B-side translates to a nearly 0 V low on the A-side, which accommodates smaller voltage swings
of lower-voltage logic. The output pulldown on the A-side drives a hard low, and the input level is
set at 0.3 × VCCA to accommodate the need for a lower low level in systems
where the low-voltage-side supply voltage is as low as 0.9 V.
The A-side of two or more TCA9517 s can be connected together, allowing many topographies (See
Figure 8 and
Figure 9 ), with the A-side as the common bus.
Also, the A-side can be connected directly to any other buffer with static- or dynamic-offset
voltage. Multiple TCA9517 s can
be connected in series, A-side to B-side, with no buildup in offset voltage and with only
time-of-flight delays to consider. The TCA9517 cannot be connected B-side to B-side, because of the buffered low
voltage from the B-side. The B-side cannot be connected to a device with rise time accelerators.
VCCA is only used to provide the 0.3 × VCCA reference to the
A-side input comparators and for the power-good-detect circuit. The TCA9517 logic and all I/Os are
powered by the VCCB pin.
As with the standard I2C system, pullup resistors are required
to provide the logic-high levels on the buffered bus. The TCA9517
has standard open-drain configuration of the
I2C bus. The size of these pullup resistors depends on the system, but
each side of the repeater must have a pullup resistor. The device is designed to work with Standard
mode and Fast mode I2C devices in addition to SMBus devices. Standard
mode I2C devices only specify 3 mA in a generic
I2C system, where Standard mode devices and multiple masters are
possible. Under certain conditions, higher termination currents can be used.
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