
11
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:
I
SS
where
I
SS
 - soft-start current (typically 10
μ
A)
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 0.1
μ
F 
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 10
μ
A 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. 
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:
, where
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 ~1
μ
s).
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 10k
, which may work fine in some 
systems. However, some of the newer systems may require 
a faster rise time than allowed by the 10k
 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 
I
COUT
BG
-----------------------------
 Σ
 C
OUT
V
OUT
×
(
)
×
,
=
C
FIGURE 10. VID_PG DELAY DEPENDENCE 
ON VID_CT CAPACITOR
0
10
20
30
40
50
60
70
80
VID_PG DELAY (ms)
0
1
2
3
4
5
6
7
8
9
10
C
t
--------------------
=
ISL6505