Data Sheet

PCA9512A_1 © Koninklijke Philips Electronics N.V. 2005. All rights reserved.
Product data sheet Rev. 01 — 7 October 2005 7 of 22
Philips Semiconductors
PCA9512A
Level shifting hot swappable I
2
C-bus and SMBus bus buffer
rate is slow enough that the output catches up it will still lag the falling voltage of the input
by the offset voltage. The maximum t
PHL
occurs when the input is driven LOW with zero
delay and the output is still limited by its turn-on delay and the falling edge slew rate. The
output falling edge slew rate is a function of the internal maximum slew rate which is a
function of temperature, V
CC
or V
CC2
and process, as well as the load current and the load
capacitance.
8.5 Rise time accelerators
During positive bus transactions, a 2 mA current source is switched on to quickly slew the
SDA and SCL lines HIGH once the input level of 0.6 V for the PCA9512A is exceeded.
The rising edge rate should be at least 1.25 V/µs to guarantee turn on of the accelerators.
8.6 ACC boost current enable
Users having lightly loaded systems may wish to disable the rise time accelerators.
Driving this pin to ground turns off the rise time accelerators on all four SDAn and SCLn
pins. Driving this pin to the V
CC2
voltage enables normal operation of the rise time
accelerators.
8.7 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 SDAn and SCLn pins, in order to activate the boost pull-up currents
during rising edges. Choose maximum resistor value using the formula:
where R
PU
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.
In addition, regardless of the bus capacitance, always choose R
PU
≤ 16 kΩ for
V
CC
= 5.5 V maximum, R
PU
≤ 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 Figure 5 and Figure 6 for guidance in resistor pull-up selection.
R
PU
800 10
3
V
CC min()
0.6–
C
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