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The design guidelines for power sequencing when using PI5C680X 1
AN32 04/24/01
Introduction
The PI5C6800 and PI5C6801 (Figure 1.) are 10-bit bus switches with
low on-state resistance, ultra low quiescent power (0.2uA typical)
and are hot swappable.
The design guidelines for power sequencing
when using PI5C680X
By Paul Li
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Application Note 32
Figure 1. The logic diagram of PI5C6800 and PI5C6801
TheTwo Power Sequencing Conditions
Figure 2. The Power Sequencing Protection Circuit using Resistor
1. The PI5C680X chip has two pins connected to power, the V
CC
and
BIASV pins. The V
CC
pin will power up the chip. The BIASV pin will
provide the bias voltage for the internal pull-up resistor
on the BX side (see Figure 2.).In normal conditions, on power up, the
V
CC
and V-bias should reach the PI5C680X at the same time. If the
V
DD
reaches the BIASV pin earlier than the power reaches the V
CC
pin (see the circuit A, Figure 2), the V
DD
at pin BIASV will shorten
to ground through the V
CC
pin and will damage the internal circuit
at the pin BIASV. Adding a 100ohm resistor connected between
V
DD
and pin BIASV will limit the current and prevent this potential
problem (see circuit B, Figure 2).
2. If the logic signals on BX come earlier than the V
CC
power on
PI5C680X, the signals on BX will shorten to ground through theV
CC
pin, and could damage the internal circuit at BX. If the BX pins are
connected to a pull-up bus (PCI bus for example), the situation
discussed above will affect the logic status of that bus. Adding a
diode on the V
CC
pin in Figure 3. will prevent this potential problem.
Figure 3. The Power sequencing protection circuit using
Germanium diode for PI5C6800 and PI5C6801
The circuit in Figure 3. will protect both of the power sequencing
problems above. Therefore, the circuit in Figure 2. is not needed if
using the circuit in Figure 3.
VCC
ON
A1
B1
Switch
Control
A10
GND
B10
BIASV
C1 0.47uf
Circuit A
PI5C6800
PI5C6801
VCC
VCC
VDD
V
CC
ON
A1
Switch
Control
A10
GND
B10
BIASV
C1 0.47uf
Circuit B
PI5C6800
PI5C6801
VCC
VDD
B1
R1
100ohm 12345678901234567890123456789012123456789012345678901234567890121234567890123456789012345678901212345678901234567890123456789012123456789012
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Application Note 32
2
AN32 04/24/01
Circuit Analysis for Figure 3.
•
A germanium diode with low forwarding voltage drop (0.2V 0.3V)
is suggested to minimize the V
CC
voltage drop. A Schottky diode
with 0.3V to 0.4V forwarding voltage drop could be an alternative.
•
R1 in Figure 3. is to reduce the V
CC
drop on D1. It will reduce the
V
CC
drop to 0.005V when quiescent mode. The voltage drop on
D1 is 0.2V when the switching frequency at pin “/on” is lower
than 10mhz.
•
C1 and C2 in Figure 3. must be as close to the V
CC
and GND pins
of PI5C6801 as possible to minimize the V
CC
ripple and ground
bounce.
•
Figure 4. and table-1 are the curves and the data to explain the
relation between the V
CC
drop on D1/R1 versus the switching
frequency at pin “/on”.
Figure 4. The Voltage Drop on D1 / R1 versus Frequency
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Table 1. The Comparison of PI5C6800 and PI5C6801: the PI5C6801 has less
I
CC
current and voltage drop on D1 / R1 at higher frequency range.
Conclusion
If the V
DD
voltage at pin BIASV comes earlier than the V
CC
voltage
at the V
CC
pin; or if the signals at BX come earlier than the V
CC
voltage at V
CC
pin, a diode as in Figure 3. is needed to prevent the
shorten current through V
CCD
pin to ground.