PCA9509P datasheet(7/23 Pages) NXP

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PCA9509P

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Product data sheet

Rev. 1 — 14 August 2012

7 of 23

NXP Semiconductors

PCA9509P

Low power level translating I2C-bus/SMBus repeater

is high the current should be set near the maximum current drive for the weakest part.

However, if the bus capacitance is low a lower current/higher resistor value should be

used to keep the rise time from getting so fast that it causes problems. The A side pull-up

resistor must be selected so as to keep the LOW-level voltage at the A side input below

0.1VCC(A).

6.4.1 Port A pull-up resistor sizing

When selecting the pull-up resistor for the A side of the PCA9509P there are several

considerations and limitations from both the PCA9509P and the other part(s) on the

A-side bus that need to be taken into account.

6.4.1.1

Minimum resistor size

The first limitation is that in order for the PCA9509P to recognize a LOW on the A side, the

voltage level must be below 0.1VCC(A) including ground offset. This and the drive strength

of the parts driving the less than 0.1VCC(A) level define the minimum resistor value that

can be used based on the other parts on the A-side bus. There is also a limit of 4 mA

maximum current that can be applied to the PCA9509P A side when it is LOW. For

example, if the part driving the PCA9509P A side is rated at 3 mA at 0.4 V (and it is a

MOS part) and assuming that VCC(A) = 0.8 V, then the part has an effective output

resistance of 0.4 V / 3 mA = 133

, so the maximum current is 0.08 V / 133  =~600 A,

so the maximum pull-up current would be 600

A. And for V

CC(A) = 0.8 V the minimum

resistance would be 0.72 V / 600

A=1.2k. If the part providing the 0.1V

CC(A) has a

very low output resistance, then the current is limited to 4 mA by the PCA9509P, and for

VCC(A) = 0.8 V the minimum pull-up resistance would be 0.64 V / 4 mA = 160 . Since the

ground offset will never be zero, the minimum resistor value will need to be increased

accordingly.

6.4.1.2

Maximum resistance sizing

The pull-up resistor on the A side of the PCA9509P should be chosen to provide at least

100

A of pull-up current. So for V

CC(A) = 0.8 V the maximum resistor value looks like

0.64 V / 100

A=6.4k.

6.4.1.3

Rise time constraints

In addition to the current minimum and maximum considerations, the RC time constant of

the A-side bus must be considered. It is recommended that the RC time constant be

chosen so that it is greater than 60 ns in order to control the size of the bounce that results

when a driver on the A side turns off that was driving a <0.1VCC(A) level and before the

offset output driver in the PCA9509P can turn on and hold the voltage to ~0.2VCC(A). The

maximum RC time constant is limited by the I2C-bus family specification for maximum

rise time between 0.3VCC(A) and 0.7VCC(A) (400 ns for Fast mode and 1000 ns for

Standard mode operation). Although the maximum current and maximum rise time limits

predict an allowable capacitance well above the 400 pF I2C-bus family specification, this

is not likely to be possible in a typical I2C-bus system since there is so little voltage margin

for the VIL of the PCA9509P ground offset is more likely to limit the effective capacitance

than current drive capability because high-capacitance buses tend to be physically large

and large size tends to result in larger ground offset voltages, which must be severely

limited to be successful at a bus voltage of 1 V and below.

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