ISL6505 Datasheet, PDF (11/17 Pages) Intersil Corporation – Multiple Linear Power Controller with ACPI Control Interface

ISL6505
Application Guidelines
Soft-Start Interval
The 5V_SB output of a typical ATX supply is capable of
725mA, with newer models rated for 1.0A, and even 2.0A.
During power-up in a sleep state, the 5V_SB ATX output
needs to provide sufficient current to charge up all the
applicable output capacitors and, simultaneously, provide
some amount of current to the output loads. Drawing
excessive amounts of current from the 5V_SB output of the
ATX can lead to voltage collapse and induce a pattern of
consecutive restarts with unknown effects on the system’s
behavior or health.
The built-in soft-start circuitry allows tight control of the slew-
up speed of the output voltages controlled by the ISL6505,
thus enabling power-ups free of supply drop-off events.
Since the outputs are ramped up in a linear fashion, the
current dedicated to charging the output capacitors can be
calculated with the following formula:
Σ I_COUT
=
-----------^I--^S----^S------------
C_SS × V_BG
×
(C_OUT × V_OUT),
where
I_SS - soft-start current (typically 10µA)
C_SS - soft-start capacitor
V_BG - bandgap voltage (typically 1.26V)
Σ(C_OUT x V_OUT) - sum of the products between the
capacitance and the voltage of an output (total charge
delivered to all outputs)
Due to the various system timing events and their
interaction, it is recommended that the soft-start interval not
be set to exceed 30ms. For most applications, a 0.1µF
capacitor is recommended.
Shutdown
In case of a FAULT condition that might endanger the
computer system, or at any other time, all the ISL6505 outputs
can be shut down by pulling the SS pin below the specified
shutdown level (typically 0.8V) with an open drain or open
collector device capable of sinking a minimum of 2mA. Pulling
the SS pin low effectively shuts down all the pass elements.
Upon release of the SS pin, the ISL6505 undergoes a new
soft-start cycle and resumes normal operation in accordance
to the ATX supply and control pins status.
VID_PG Delay
During power-up and initial soft-start, the VID_PG and
VID_CT pins are held low. As the 1V2VID output exceeds its
rising power-good threshold (typically 90% of its final value),
the capacitor connected at the VID_CT pin starts to charge
up through the internal 10µA current source. As the voltage
on this capacitor exceeds 1.25V, the open-collector VID_PG
pin is released and VID POWER GOOD status is thus
reported.
80
70
60
50
40
30
20
10
0
01 2 3 4 5 6 7 8
VID_PG DELAY (ms)
FIGURE 10. VID_PG DELAY DEPENDENCE
ON VID_CT CAPACITOR
9 10
The value of the VID_CT capacitor to be used to obtain a
given VID_PG delay can be determined from the graph in
Figure 10. For extended delays exceeding the range of the
graph, use the following formula:
C = ^t--^D-----^E----^L---^A----^Y-- ^{, where}
125000
t_DELAY - desired delay time (s)
C - VID_CT capacitor to obtain desired delay time (F)
If no delay is needed, then a very small (pF) capacitor, or
even no capacitor at all will generate a very short delay (just
the pin capacitance of ~10pF should give a delay of ~1µs).
The value of the external VID_PG pull-up resistor is
determined by the trade-off between the pull-down current
available from the pin versus the rise time needed. In the
typical power-up sequence (as described above), the
VID_PG starts low (VID Power NOT Good) until the 1V2VID
output reaches its power-good threshold (90%), which starts
the VID_CT pin charging. When that pin reaches its trip
point, the VID_PG pin open-drain pull-down device shuts off,
and the external pull-up resistor (R2, as shown in Figure 13)
will pull the output up to the positive supply (typically
1V2VID). This rise time is determined not by the ISL6505,
but simply by the RC time constant of the pull-up resistor,
and whatever capacitance is on the node, from the VID_PG
output pin to whatever signals it is driving, including the pin
capacitances and all of the parasitics; this may vary from
one system implementation to another.
The R2 value in Figure 13 (and on the ISL6505EVAL1/2
boards) is listed as 10kΩ, which may work fine in some
systems. However, some of the newer systems may require
a faster rise time than allowed by the 10kΩ resistor, so a
lower value of resistance should be chosen. But the VID_PG
pin must be able to pull down low enough against the
resistor to guarantee a low logic level for whatever control
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