9

FN9062.2

April 13, 2004

Soft-Start into Sleep States (S3, S4/S5)

internal 10

µA current source charges an external capacitor.

The error amplifiers reference inputs are clamped to a level

proportional to the SS (soft-start) pin voltage. As the SS pin

voltage slews from about 1.25V to 2.5V, the input clamp

allows a rapid and controlled output voltage rise.

Figures 7 (ISL6504) and 8 (ISL6504A) show the soft-start

sequence for the typical application start-up into a sleep

circuit quickly raises the SS capacitor voltage to

approximately 1V, then the 10

µA current source continues

the charging.

The soft-start capacitor voltage reaches approximately

transition, resulting in the output voltages ramping up

proportionally. The ramp-up continues until time T3 when the

two voltages reach the set value. As the soft-start capacitor

voltage reaches approximately 2.75V, the undervoltage

monitoring circuit of this output is activated and the soft-start

capacitor is quickly discharged to approximately 1.25V.

Following the 3ms (typical) time-out between T3 and T4, the

soft-start capacitor commences a second ramp-up designed

to smoothly bring up the remainder of the voltages required

by the system. At time T5, voltages are within regulation

limits, and as the SS voltage reaches 2.75V, all the

remaining UV monitors are activated and the SS capacitor is

quickly discharged to 1.25V, where it remains until the next

active state, it does not come up, but rather waits until the

main ATX outputs come up within regulation limits.

Soft-Start into Active States (S0, S1)

applied, the ISL6504/A will assume active state wake-up and

keep off the required outputs until some time (typically

25ms) after the monitored main ATX output (3.3V) exceeds

the set threshold. This time-out feature is necessary in order

to ensure the main ATX outputs are stabilized. The time-out

also assures smooth transitions from sleep into active when

surpasses POR level.

0V

0V

TIME

SOFT-START

(1V/DIV)

OUTPUT

(1V/DIV)

VOLTAGES

VOUT1 (1.5VSB)

VOUT4 (5VDUAL) IF S3

T1 T2

T3

T0

5VSB

(1V/DIV)

T5

T4

VOUT3 (3.3VDUAL/3.3VSB)

VOUT2

(1.2VVID)

VOUT4 (5VDUAL) if S5

FIGURE 7. SOFT-START INTERVAL IN A SLEEP

STATE; ISL6504

0V

0V

SOFT-START

(1V/DIV)

OUTPUT

(1V/DIV)

VOLTAGES

VOUT1 (1.5VSB)

VOUT4 (5VDUAL)

T1 T2

T3

T0

5VSB

(1V/DIV)

T5

T4

VOUT3 (3.3VDUAL/3.3VSB)

VOUT2

(1.2VVID)

FIGURE 8. SOFT-START INTERVAL IN A SLEEP

STATE; ISL6054A

TIME

ISL6504, ISL6504A