11

Application Guidelines

Soft-Start Interval

725mA, with newer models rated for 1.0A, and even 2.0A.

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

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:

where

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

µF

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

µA 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

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

µs).

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

Ω, which may work fine in some

systems. However, some of the newer systems may require

a faster rise time than allowed by the 10k

Ω 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

I

SS

C

SS

V

BG

×

------------------------------

OUT

V

OUT

×

()

×

,

=

FIGURE 10. VID_PG DELAY DEPENDENCE

ON VID_CT CAPACITOR

0

10

20

30

40

50

60

70

80

VID_PG DELAY (ms)

0

12

3

4

56

789

10

C

t

DELAY

125000

--------------------

=

ISL6505