Philips Semiconductors Product data
PCA9510; PCA9511
Hot swappable I
2
C and
SMBus bus buffer
2003 Dec 18
7
Resistor Pull-up Value Selection
The system pull-up resistors must be strong enough to provide a
positive slew rate of 1.25 V/µs on the SDA and SCL pins, in order to
activate the boost pull-up currents during rising edges. Choose
maximum resistor value using the formula:
R v 800 @ 10
3
V
CC(MIN)
* 0.6
C
where R is the pull-up resistor value in Ω, V
CC(MIN)
is the
minimum V
CC
voltage in volts and C is the equivalent bus
capacitance in picofarads (pF).
In addition, regardless of the bus capacitance, always choose R ≤
16 kΩ for V
CC
= 5.5 V maximum, R ≤ 24 kΩ for V
CC
= 3.6 V
maximum. The start-up circuitry requires logic high voltages on
SDAOUT and SCLOUT to connect the backplane to the card, and
these pull-up values are needed to overcome the precharge voltage.
See the curves in Figures 6 and 7 for guidance in resistor pull-up
selection.
30
20
15
10
5
0
0 100 200 300 400
C
BUS
(pF)
R
PULLUP
(kΩ)
25
RECOMMENDED
PULL-UP
R
MAX
= 24 k
RISE-TIME > 300 NS
SW02115
Figure 6. Bus requirements for 3.3 V systems
20
15
10
5
0
0 100 200 300 400
C
BUS
(pF)
R
PULLUP
(kΩ)
RECOMMENDED
PULL-UP
R
MAX
= 16 k
RISE-TIME
> 300 NS
SW02116
Figure 7. Bus requirements for 5 V systems
Minimum SDA and SCL Capacitance
Requirements
The device connection circuitry requires a minimum capacitance
loading on the SDA and SCL pins in order to function properly. The
value of this capacitance is a function of V
CC
and the bus pull-up
resistance. Estimate the bus capacitance on both the backplane and
the card data and clock buses, and refer to Figures 6 and 7 to
choose appropriate pull-up resistor values. Note from the figures
that 5 V systems must have at least 47 pF capacitance on their
buses and 3.3 V systems must have at least 22 pF capacitance for
proper operation. For applications with less capacitance, add a
capacitor to ground to ensure these minimum capacitance
conditions.
Hot Swapping and Capacitance Buffering
Application
Figures 8 through 11 illustrate the usage of the PCA9510 and
PCA9511 in applications that take advantage of both its hot
swapping and capacitance buffering features. In all of these
applications, note that if the I/O cards were plugged directly into the
backplane, all of the backplane and card capacitances would add
directly together, making rise- and fall-time requirements difficult to
meet. Placing a bus buffer on the edge of each card, however,
isolates the card capacitance from the backplane. For a given I/O
card, the PCA9510 and PCA9511 drive the capacitance of
everything on the card and the backplane must drive only the
capacitance of the bus buffer, which is less than 10 pF, the
connector, trace, and all additional cards on the backplane.
See Application Note AN10160, Hot Swap Bus Buffer for more
information on applications and technical assistance.
