PCA9517DP datasheet(3/13 Pages) PHILIPS

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Philips Semiconductors

Product data sheet

PCA9517

Level translating I2C-bus repeater

2004 Oct 05

BLOCK DIAGRAM

PCA9517

VCCB

SDAB

SCLB

SCLA

SDAA

SU01791

PULL-UP

RESISTOR

GND

VCCA

VCCB

Figure 2. PCA9517 block diagram

The output pull-down on the B side internal buffer LOW is set for

approximately 0.5 V, while the input threshold of the internal buffer is

set about 70 mV lower (0.43 V). When the B side I/O is driven LOW

internally, the LOW is not recognized as a LOW by the input. This

prevents a lock-up condition from occurring. The output pull-down

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.

FUNCTIONAL DESCRIPTION

The PCA9517 enables I2C-bus or SMBus translation down to VCCA

as low as 0.9 V without degradation of system performance. The

PCA9517 contains two bi-directional, open drain buffers specifically

designed to support up-translation/down-translation between the low

voltage (as low as 0.9 V ) and a 3.3 V or 5 V I2C-bus or SMBuses.

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 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 enable (EN) HIGH, a LOW level on the A side (below

0.3VCCA) turns the corresponding B side driver (either SDA or SCL)

on and drives the B side down to about 0.5 V. When the A side rises

above 0.3VCCA the B side pull-down driver is turned off and the

external pull-up resistor pulls the pin HIGH. When the B side falls

first and goes below 0.3VCCB the A side driver is turned on and the

A side pulls down to 0 V. The B side pull-down 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 will turn off when the B side

voltage is above 0.7VCCB. If the B side low voltage goes below

0.4 V, the B side pull-down driver is enabled and the B side will only

be able to rise to 0.5 V until the A side rises above 0.3VCCA, then

the B side will continue to rise being pulled up by the external pull-up

resistor. The VCCA is only used to provide the 0.3VCCA 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.

Enable

The EN pin is active-HIGH with an internal pull-up to VCCB and

allows the user to select when the repeater is active. This can be

used to isolate a badly behaved slave on power-up until after the

system power-up reset. It should never change state during an I2C

operation because disabling during a bus operation will hang the

bus and enabling part way through a bus cycle could confuse the

I2C parts being enabled.

The enable pin should only change state when the global bus and

the repeater port are in an idle state to prevent system failures.

I2C Systems

As with the standard I2C 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 devices in addition to SMBus

devices. Standard mode I2C devices only specify 3 mA output drive,

this limits the termination current to 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.

Please see Application Note AN255

“I2C & SMBus Repeaters, Hubs

and Expanders” for additional information on sizing resistors and

precautions when using more than one PCA9517 in a system or

using the PCA9517 in conjunction with other bus buffers.

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