PCA9510A_4 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 04 — 18 August 2009 7 of 24
NXP Semiconductors
PCA9510A
Hot swappable I
2
C-bus and SMBus bus buffer
8.4 Propagation delays
The delay for a rising edge is determined by the combined pull-up current from the bus
resistors and the rise time accelerator current source and the effective capacitance on the
lines. If the pull-up currents are the same, any difference in rise time is directly
proportional to the difference in capacitance between the two sides. The t
PLH
may be
negative if the output capacitance is less than the input capacitance and would be positive
if the output capacitance is larger than the input capacitance, when the currents are the
same.
The t
PHL
can never be negative because the output does not start to fall until the input is
below 0.7V
CC
, and the output turn on has a non-zero delay, and the output has a limited
maximum slew rate, and even if the input slew 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
and process, as
well as the load current and the load capacitance.
8.5 READY digital output
This pin provides a digital flag which is LOW when either ENABLE is LOW or the start-up
sequence described earlier in this section has not been completed. READY goes HIGH
when ENABLE is HIGH and start-up is complete. The pin is driven by an open-drain
pull-down capable of sinking 3 mA while holding 0.4 V on the pin. Connect a resistor of
10 kΩ to V
CC
to provide the pull-up.
8.6 ENABLE low current disable
Grounding the ENABLE pin disconnects the backplane side from the card side, disables
the rise time accelerators, drives READY LOW, disables the bus precharge circuitry, and
puts the part in a low current state. When the pin voltage is driven all the way to V
CC
, the
part waits for data transactions on both the backplane and card sides to be complete
before reconnecting the two sides.
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 Equation 1:
(1)
where R
PU
is the pull-up resistor value in ohms, 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
≤ 65.7 kΩ for
V
CC
= 5.5 V maximum, R
PU
≤ 45 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
-----------------------------------
≤
