13

the supplied outputs. At the transition between active and

sleep states, this phenomena could result in the 5VSB

voltage dropping below the POR level (typically 4.1V) and

temporarily disabling the HIP6500. The solution to a

potential problem such as this is using larger input

capacitors with a lower total combined ESR.

Transistor Selection/Considerations

The HIP6500 usually requires one P-Channel (or bipolar

PNP), two N-Channel MOSFETs and two bipolar NPN

transistors.

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/switching element is

Select a package and heatsink that maintains the junction

temperature below the rating with the maximum expected

ambient temperature.

Q1

different requirements for each of the two voltage settings. In

2.5V systems utilizing RDRAM (or voltage-compatible)

memory, Q1 has to be a bipolar NPN capable of conducting

is actively regulated while in active state. In 3.3V systems

(SDRAM or compatible) Q1 has to be an N-Channel

MOSFET; in such systems the MOSFET is switched on

during active state (S0, S1). The main criteria for the

selection of this transistor is output voltage budgeting. The

can be expressed with the following equation:

, where

The gate bias available for this MOSFET is of the order of 8V.

Q5

If a P-Channel MOSFET is used to switch the 5VSB output

states (as dictated by EN5VDL status), then, similar to the

situation where Q1 is a MOSFET, the selection criteria of this

device is also proper voltage budgeting. The maximum

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).

Q3,4

The two N-Channel MOSFETs are used to switch the 3.3V

and 5V inputs provided by the ATX supply into the

3.3VDUAL and 5VDUAL outputs, respectively, while in active

as well. Unlike the PMOS, however, these NMOS transistors

and 7V, respectively).

Q2

The NPN transistor used as sleep state pass element (Q2)

operating temperature range.

P

LINEAR

I

O

V

IN

V

OUT

–

()

×

=

r

DS ON

()max

V

INmin

V

OUTmin

–

I

OUTmax

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

=

HIP6500