PCA9509 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 7 — 4 November 2014 5 of 24
NXP Semiconductors
PCA9509
Level translating I
2
C-bus/SMBus repeater
6. Functional description
Refer to Figure 1 “Functional diagram of PCA9509”.
The PCA9509 enables I
2
C-bus or SMBus translation down to V
CC(A)
as low as 1.35 V
without degradation of system performance. The PCA9509 contains 2 bidirectional
open-drain buffers specifically designed to support up-translation/down-translation
between the low voltage and 3.3 V SMBus or 5 V I
2
C-bus. 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
V
CC(B)
is above 2.5 V and the V
CC(A)
is above 0.8 V. V
CC(B)
and V
CC(A)
can be applied in
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 on the corresponding port B driver (either SDA
or SCL) 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
pin HIGH. When port B falls first and goes below 0.3V
CC(B)
, the port A driver is turned on
and port A pulls down to 0.2 V (typical). The port B pull-down is not enabled unless the
port A voltage goes below V
ILc
. If the port A low voltage goes below V
ILc
, the port B
pull-down driver is enabled until port A rises above approximately 0.15 V (V
ILc
), then
port B, if not externally driven LOW, continues 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 has an output resistance of 133 or less (R = E / I). Such
a driver shares 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 than
0 V, then the driver resistance must be less. Since V
ILc
can be as low as 90 mV at 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
the PCA9509 is placed in an application where the V
IL
of port A of the PCA9509 does not
go below its V
ILc
, it pulls port B LOW initially when port A input transitions LOW, but the
port B returns HIGH, so it does not reproduce the port A input on port B. Such applications
should be avoided.
Port B is interoperable with all I
2
C-bus slaves, masters and repeaters.
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
reset. It should never change state during an I
2
C-bus operation because disabling during
a bus operation hangs the bus and enabling part way through a bus cycle could confuse
the I
2
C-bus parts being enabled.
The enable pin should only change state when the bus and the repeater port are in an idle
state to prevent system failures.
