
10
FN9062.2
April 13, 2004
During sleep-to-active state transitions from conditions 
where the 5V
DUAL 
output is initially 0V (such as S5 to S0 
transition, or simple power-up sequence directly into active 
state), the circuit goes through a quasi soft-start, the 
5V
DUAL 
output
being pulled high through the body diode of 
the N-Channel MOSFET connected between it and the 5V 
ATX. Figure 9 exemplifies this start-up case. 5V
SB
 is already 
present when the main ATX outputs are turned on, at time 
T0. As a result of +5V
IN
 ramping up, the 5V
DUAL
 output 
capacitors charge up through the body diode of Q4 (see 
Typical Application). At time T1, all main ATX outputs 
exceed the ISL6504/A’s undervoltage thresholds, and the 
internal 25ms (typical) timer is initiated. At T2, the time-out 
initiates a soft-start, and the 1.2V voltage ID output is 
ramped-up, reaching regulation limits at time T3. 
Simultaneous with the beginning of this ramp-up, at time T2, 
the DLA pin is released, allowing the pull-up resistor to turn 
on Q2 and Q4, and bring the 5V
DUAL
 output in regulation. 
Shortly after time T3, as the SS voltage reaches 2.75V, the 
soft-start capacitor is quickly discharged down to 
approximately 2.45V, where it remains until a valid sleep 
state request is received from the system.
Fault Protection
All the outputs are monitored against undervoltage events. A 
severe overcurrent caused by a failed load on any of the 
outputs, would, in turn, cause that specific output to 
suddenly drop. If any of the output voltages drops below 
80% (typical) of their set value, such event is reported by 
having the FAULT pin pulled to 5V. Additionally, exceeding 
the maximum current rating of an integrated regulator 
(output with pass regulator on chip) can lead to output 
voltage drooping; if excessive, this droop can ultimately trip 
the undervoltage detector and send a FAULT signal to the 
computer system.
A FAULT condition occurring on an output when controlled 
through an external pass transistor will only set off the 
FAULT flag, and it will not shut off or latch off any part of the 
circuit. A FAULT condition occurring on an output controlled 
through an internal pass transistor, will set off the FAULT 
flag, and it will shut off the respective faulting regulator only. 
If shutdown or latch off of the entire circuit is desired in case 
of a fault, regardless of the cause, this can be achieved by 
externally pulling or latching the SS pin low. Pulling the SS 
pin low will also force the FAULT pin to go low and reset any 
internally latched-off output.
Special consideration is given to the initial start-up 
sequence. If, following a 5V
SB
 POR event, any of the 
1.5V
SB
 or 3.3V
DUAL
/3.3V
SB
 outputs is ramped up and is 
subject to an undervoltage event before the end of the 
second soft-start ramp, then the FAULT output goes high 
and the entire IC latches off. Latch-off condition can be reset 
by cycling the bias power (5V
SB
). Undervoltage events on 
the 1.5V
SB
 and the 3.3V
DUAL
/3.3V
SB
 outputs at any other 
times are handled according to the description found in the 
second paragraph under the current heading.
Another condition that could set off the FAULT flag is chip 
overtemperature. If the ISL6504/A reaches an internal 
temperature of 140
o
C (typical), the FAULT flag is set, but the 
chip continues to operate until the temperature reaches 
155
o
C (typical), when unconditional shutdown of all outputs 
takes place. Operation resumes only after powering down 
the IC (to create a 5V
SB
 POR event) and a start-up 
(assuming the cause of the fault has been removed; if not, 
as it heats up again, it will repeat the FAULT cycle).
In ISL6504/A applications, loss of the active ATX output 
(3.3V
IN
; as detected by the on-board voltage monitor) during 
active state operation causes the chip to switch to S5 sleep 
state, in addition to reporting the input UV condition on the 
FAULT pin. Exiting from this forced S5 state can only be 
achieved by returning the faulting input voltage above its UV 
threshold, by resetting the chip through removal of 5V
SB
bias voltage, or by bringing the SS pin at a potential lower 
than 0.8V.
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 
FIGURE 9. SOFT-START INTERVAL IN ACTIVE STATE
0V
0V
TIME
OUTPUT
VOLTAGES
(1V/DIV)
T1
T2
T3
T0
INPUT VOLTAGES
(2V/DIV)
+5VIN
+12VIN
+5VSB
VOUT1 (1.5VSB)
VOUT3 (3.3VDUAL/3.3VSB)
VOUT4 (5VDUAL)
DLA PIN
(2V/DIV)
+3.3VIN
VOUT2 (1.2VVID)
SOFT-START
(1V/DIV)
ISL6504, ISL6504A