7

FN9141.6

September 12, 2013

current demands. Thus, it is recommended that the output

capacitors be selected for transient load regulation, paying

attention to their parasitic components (ESR, ESL).

Also, during the transition between active and sleep states

which none of the power pass elements are conducting.

During this time the output capacitors have to supply all the

output current. The output voltage drop during this brief

period of time can be easily approximated using Equation 1:

where:

ΔV

typical)

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 ISL6506, ISL6506A 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 ISL6506, ISL6506A’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

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 ISL6506, ISL6506A usually requires one P-Channel and

two N-Channel MOSFETs. All three of these MOSFETs are

utilized as ON/OFF switching elements.

One important criteria for selection of transistors for all the

switching elements is package selection for efficient removal

of heat. The power dissipated in a switch element while on is

shown in Equation 2:

Select a package and heatsink that maintains the junction

temperature below the rating with the maximum expected

ambient temperature.

Q1, Q3

These N-Channel MOSFETs are used to switch the 3.3V and

criteria for the selection of these transistors is output voltage

temperature can be expressed using Equation 3:

where:

Q2

sleep states. The selection criteria of this device, as with the

N-Channel MOSFETs, is proper voltage budgeting. The

4.5V of gate-to-source voltage, so a true logic level

MOSFET needs to be selected.

V

OUT

Δ

I

OUT

ESR

OUT

t

t

C

OUT

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

+

⎝⎠

⎜⎟

⎛⎞

×

=

(EQ. 1)

P

LOSS

2

()

×

=

(EQ. 2)

r

DS ON

()max

V

INmin

V

OUTmin

–

I

OUTmax

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

=

(EQ. 3)

ISL6506, ISL6506A, ISL6506B