
14
The output voltage drop is heavily dependent on the ESR 
(equivalent series resistance) of the output capacitor bank, 
the choice of capacitors should be such as to maintain the 
output voltage above the lowest allowable regulation level.
Input Capacitors Selection
The input capacitors for an ISL6505 application must have a 
sufficiently low ESR so as not to allow the input voltage to 
dip excessively when energy is transferred to the output 
capacitors. If the ATX supply does not meet the 
specifications, certain imbalances between the ATX’s 
outputs and the ISL6505’s regulation levels could have as a 
result a brisk transfer of energy from the input capacitors to 
the supplied outputs. At the transition between active and 
sleep states, such phenomena could be responsible for the 
5V
SB
 voltage drooping excessively and affecting the output 
regulation. The solution to such a potential problem is using 
larger input capacitors with a lower total combined ESR.
Transistor Selection/Considerations
The ISL6505 usually requires one P-Channel (or bipolar 
PNP), three N-Channel MOSFETs, and one bipolar NPN 
transistors. Note there is no Q1 listed below.
One important criteria for selection of transistors for all the 
linear regulators/switching elements is package selection for 
efficient removal of heat.
The power dissipated in a linear regulator or an ON/OFF 
switching element is 
(
)
×
=
Select a package and heatsink that maintains the junction 
temperature below the rating with the maximum expected 
ambient temperature.
Q2
The NPN transistor used as sleep state pass element on the 
3.3V
DUAL
 output has to have a minimum current gain of 100 
at 1.5V V
CE
 and 650mA I
CE
 throughout the in-circuit 
operating temperature range. For larger current ratings on 
the 3.3V
DUAL
 output (providing the ATX 5V
SB
 output rating 
is equally extended), selection criteria for Q2 include an 
appropriate current gain (h
fe
) and saturation characteristics.
Q3, Q5
These NMOS FETs are used to switch the 3.3V and 5V inputs 
provided by the ATX supply into the 3.3V
DUAL
/3.3V
SB
 and 
5V
DUAL
 outputs while in active (S0, S1) state. The main 
criteria for the selection of these transistors is output voltage 
budgeting. The maximum r
DS(ON)
 allowed at highest junction 
temperature can be expressed with the following equation:
V
–
OUTmax
, where
V
INmin
 - minimum input voltage
V
OUTmin
 - minimum output voltage allowed
I
OUTmax
 - maximum output current
Q4
If a PMOS FET is used to switch the 5V
SB
 output of the ATX 
supply into the 5V
DUAL
 output during sleep states, then the 
selection criteria of this device is proper voltage budgeting. 
The maximum r
DS(ON)
, however, has to be achieved with 
only 4.5V of gate-to-source voltage, so a logic level 
MOSFET needs to be selected. If a PNP device is chosen to 
perform this function, it has to have a low- saturation voltage 
while providing the maximum sleep current and have a 
current gain sufficiently high to be saturated using the 
minimum drive current (typically 20mA).
Q6
This NMOS FET acts as the pass transistor for the V
OUT1
output. The input voltage to the source comes from 
3.3V
DUAL
/3.3V
SB
; the output is expected to be in the 1.2V 
to 1.5V range, depending upon the external resistor divider. 
The power dissipation will (3.3V - VOUT1) * IOUT1, so the 
FET selection and mounting technique must be sufficient for 
that case.
In addition, Q6 must have a sufficiently low gate threshold 
voltage. The DR1 gate driver maximum voltage is limited to 
a V
BE
 below the 5V supply (5VSB pin), which itself can be 
as low as 4.5V. So the maximum driver voltage can be 3.8V 
(or even a few tenths of a volt lower, considering 
temperature effects of the V
BE 
drop). If the output voltage is 
1.5V, then only 2.3V is available for the gate threshold, plus 
any overdrive needed to get the required output current out 
while close to the threshold. So a FET gate threshold voltage 
well below 2V is recommended. 
Although the design intended to use a low threshold voltage 
FET for Q6, it is possible to use an NPN. The DR1 driver is 
rated at 10mA nominal, but the driver will only supply the 
current that is needed, up to 50mA; in that sense, it is more 
of a maximum spec. That means an NPN with a gain of 100 
could deliver up to 1.0A, for example. The additional base 
current will come from the 5VSB pin, which adds some IC 
power dissipation, and may take away current needed 
elsewhere; it may also draw the extra current during sleep 
modes. But the NPN can be used, and it should be stable, 
using the same considerations for a FET. 
The LAN pin determines the timing and the on state of the 
regulator; when LAN is high (5V) or open, V
OUT1 
stays on all 
of the time. The input current comes from 3.3V
DUAL
/3.3V
SB
, 
which indirectly comes from 3V3 during active modes, and 
from the 3.3V
SB
 regulator (which ultimately comes from 
5V
SB
); thus, the V
OUT1
 current is limited in sleep mode to 
whatever current the 5V
SB
 has left, after all other currents 
are accounted for. The current in active mode can be higher, 
limited mainly by the dissipation of the FET. 
When the LAN pin is low, then V
OUT1
 is on only during 
active states, where the input voltage is ultimately 3V3. So 
again, the current is limited mainly by the dissipation of the 
FET.
P
LINEAR
I
O
V
IN
V
OUT
–
r
DS ON
)
max
V
--------------------------------------------------
=
ISL6505