This dual bidirectional I2C buffer is operational at 2.7 V to
The PCA9517 is a BiCMOS integrated circuit intended for I2C
bus and SMBus systems. It can also provide 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
similar bus systems to be extended, without degradation of performance even during level shifting.
The PCA9517 buffers both the serial data (SDA) and the serial clock (SCL) signals on the
I2C bus, thus allowing two buses of 400-pF bus capacitance to be
connected in an I2C application. This device can also be used to isolate
two halves of a bus for voltage and capacitance.
The PCA9517 has two types of drivers—A-side drivers and B-side drivers. All inputs and
I/Os are overvoltage tolerant to 5.5 V, even when the device is unpowered
(VCCB and/or VCCA = 0 V).
The PCA9517 doesnot support clock stretching and arbitration across the
The B-side drivers operate from 2.7 V to 5.5 V and behave like the drivers in the
PCA9515A. 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.
This type of design on the B side prevents it from being used in series with the PCA9515A
and another PCA9517 (B side). 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 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 PCA9517s can be connected together to allow a star topography,
with the A side on the common bus. Also, the A side can be connected directly to any other buffer
with static- or dynamic-offset voltage. Multiple PCA9517s 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 PCA9517 drivers are enabled when VCCA is above 0.8 V and
VCCB is above 2.5 V.
The PCA9517 has an active-high enable (EN) input with an internal pullup to
VCCB, which allows the user to select when the repeater is active. This can
be used to isolate a badly behaved slave on power-up reset. It should never change state during an
I2C operation, because disabling during a bus operation hangs the bus,
and enabling part way through a bus cycle could confuse the I2C parts
being enabled. The EN input should change state only when the global bus and repeater port are in
an idle state, to prevent system failures.
The PCA9517 includes a power-up circuit that keeps the output drivers turned off until
VCCB is above 2.5 V and the VCCA is above 0.8 V.
VCCB and VCCA can be applied in any sequence at power
up. After power up and with the EN high, a low level on the A side (below
0.3 VCCA) turns the corresponding B-side driver (either SDA or SCL) on and
drives the B side down to approximately 0.5 V. When the A side rises above
0.3 VCCA, the B-side pulldown driver is turned off and the external pullup
resistor pulls the pin high. When the B side falls first and goes below 0.3
VCCB, the A-side driver is turned on and the A side pulls down to 0 V. The
B-side pulldown is not enabled unless the B-side voltage goes below 0.4 V. If the B-side low
voltage does not go below 0.5 V, the A-side driver turns off when the B-side voltage is above 0.7
VCCB. If the B-side low voltage goes below 0.4 V, the B-side pulldown driver
is enabled, and the B side is able to rise to only 0.5 V until the A side rises above 0.3
VCCA. Then the B side continues to rise, being pulled up by the external
pullup resistor. 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 PCA9517 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 PCA9517 has standard open-collector
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.