HIP6502B

FN4871 Rev.0.00

Page 12 of 14

May 2000

HIP6502B. The solution to a potential problem such as this is

using larger input capacitors with a lower total combined ESR.

Transistor Selection/Considerations

The HIP6502B usually requires one P-Channel (or bipolar

PNP), three 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

The active element on the 2.5VMEM output has to be a bipolar

NPN capable of conducting the maximum active memory

current and exhibit a current gain (hfe) of minimum 40 at this

current and 0.7V VCE.

Q2

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

the 3.3VDUAL output has to have a minimum current gain of

100 at 1.5V VCE and 500mA ICE throughout the in-circuit

operating temperature range.

Q3, Q4, Q6

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

and 5V inputs provided by the ATX supply into the 3.3VMEM,

3.3VDUAL/3.3VSB, and 5VDUAL outputs, while in active (S0,

S1) state. The main criteria for the selection of these

transistors is output voltage budgeting. The maximum rDS(ON)

allowed at highest junction temperature can be expressed with

the following equation:

, where

VINmin - minimum input voltage

VOUTmin - minimum output voltage allowed

IOUTmax - maximum output current

The gate bias available for these MOSFETs is of the order of

8V.

Q5

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

the ATX supply into the 5VDUAL output during S3 and S5

states (as dictated by EN5VDL status), then the selection

criteria of this device is proper voltage budgeting. The

maximum rDS(ON), however, has to be achieved with only

4.5V of VGS, 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).

PLINEAR

IO VIN VOUT

–





=

rDS ON

max

VINmin VOUTmin

–

IOUTmax

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=