PCA9512 Philips Semiconductors, PCA9512 Datasheet - Page 6

• Manufacturer Part Number: PCA9512
• Description: Level shifting hot swappable I2C and SMBus buffer
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Resistor Pull-up Value Selection
The system pull-up resistors must be strong enough to provide a
positive slew rate of 1.25 V/ s 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:
where R is the pull-up resistor value in ohms, V
minimum V
picofarads (pF).
In addition, regardless of the bus capacitance, always choose
R
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 5 and 6 for guidance in resistor pull-up
selection.
2004 Oct 05
Level shifting hot swappable I
R
R
16 k for V
PULLUP
(k )
(V
CC(MIN)
Figure 5. Bus requirements for 3.3 V systems
CC
30
25
20
15
21
5
0
voltage and C is the equivalent bus capacitance in
0
CC
– 0.6) (800,000)/C
= 5.5 V maximum, R
100
RECOMMENDED
C
BUS
PULL-UP
(pF)
RISE-TIME > 300 ns
200
24 k for V
CC(MIN)
R
MAX
300
CC
= 24 k
2
is the
= 3.6 V
C and SMBus buffer
SW02115
400
6
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
resistance. Estimate the bus capacitance on both the backplane and
the card data and clock buses, and refer to Figures 5 and 6 to
choose appropriate pull-up resistor values. Note from the figures
that 5 V systems should have at least 47 pF capacitance on their
buses and 3.3 V systems should have at least 22 pF capacitance for
proper operation of the PCA9512. Although the device has been
designed to be marginally stable with smaller capacitance loads, for
applications with less capacitance, provisions need to be made to
add a capacitor to ground to ensure these minimum capacitance
conditions if oscillations are noticed during initial signal integrity
verification.
R
PULLUP
(k )
Figure 6. Bus requirements for 5 V systems
20
15
21
5
0
0
100
RECOMMENDED
C
PULL-UP
BUS
200
(pF)
CC
and the bus pull-up
R
PCA9512
Product data sheet
MAX
300
= 16 k
RISE-TIME
> 300 ns
SW02116
400