PCA9509_5

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 05 — 10 July 2009

4 of 19

NXP Semiconductors

PCA9509

6.

Functional description

Refer to Figure 1 “Functional diagram of PCA9509”.

low as 0.95 V in special cases) without degradation of system performance. The

PCA9509 contains 2 bidirectional open-drain buffers specifically designed to support

The port B I/Os are over-voltage tolerant to 5.5 V even when the device is unpowered.

The PCA9509 includes a power-up circuit that keeps the output drivers turned off until

any sequence at power-up. After power-up and with the EN pin HIGH, a LOW level on

port A (below approximately 0.15 V) turns the corresponding port B driver (either SDA or

SCL) on and drives port B down to about 0 V. When port A rises above approximately

0.15 V, the port B pull-down driver is turned off and the external pull-up resistor pulls the

and port A pulls down to 0.2 V (typical). The port B pull-down is not enabled unless the

port B, if not externally driven LOW, will continue to rise being pulled up by the external

pull-up resistor.

Remark: Ground offset between the PCA9509 ground and the ground of devices on

port A of the PCA9509 must be avoided.

The reason for this cautionary remark is that a CMOS/NMOS open-drain capable of

sinking 3 mA of current at 0.4 V will have an output resistance of 133

Ω or less (R=E/I).

Such a driver will share enough current with the port A output pull-down of the PCA9509

to be seen as a LOW as long as the ground offset is zero. If the ground offset is greater

cold temperatures and the low end of the current distribution, the maximum ground offset

should not exceed 50 mV.

Bus repeaters that use an output offset are not interoperable with the port A of the

PCA9509 as their output LOW levels will not be recognized by the PCA9509 as a LOW. If

port B will return HIGH, so it will not reproduce the port A input on port B. Such

applications should be avoided.

6.1 Enable

The EN pin is active HIGH 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

a bus operation will hang the bus and enabling part way through a bus cycle could

The enable pin should only change state when the bus and the repeater port are in an idle

state to prevent system failures.