RT9641BGS datasheet(10/11 Pages) RICHTEK

10 / 11 page

RT9641A/B

DS9641A/B-11 March 2007

www.richtek.com

The RT9641A/B typically requires one P-channel or PNP

transistor and two N-Channel power MOSFETs and two

bipolar NPN transistors.

One general requirement 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:

PLINEAR = IO x (VIN - VOUT)

Select a package and heatsink that maintains the junction

temperature below the rating with the maximum expected

ambient temperature.

The active element on the 2.5V/3.3V (2.6V/3.43V) V(MEM)

output has different requirements for each the two voltage

settings. In 2.5V systems utilizing RDRAM (or voltage-

compatible) memory, Q1 had better to be a bipolar NPN

capable of conducting the maximum required output

current and it must have a minimum current gain (hfe) of

100~150 at this current and 0.7V VCE. In such systems,

the 2.5V(2.6V) output is regulated from the ATX 3.3V output

while in an active state. In 3.3V systems (SDRAM or

compatible) Q1 is suggested to use an N-channel

MOSFET, then the MOSFET serves like a switch when it

is connected to ATX3.3V during active states (S0, S1).

The main criteria for the selection of this transistor is output

voltage budgeting. The maximum RDS(ON) allowed at

highest junction temperature can be expressed with the

following equation:

RDS(ON) MAX = (VIN(MIN)

−VOUT (MIN))/ IOUT( MAX), where

VIN(MIN) : minimum input voltage

VOUT(MIN) : minimum output voltage allowed.

IOUT(MAX) : maximum output current

The gate bias available for this MOEFET is approximately

6V, so the logic level MOSFET is prefered. The

3.3V(3.43V) V(MEM) power also can be regulated from ATX

5V in order to have high quality V(MEM), in such a

configuration, either MOSFET or NPN transistors can be

used. While the heat dissipation should be carefully

handled.

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

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

S4/S5 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 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-state current and have current gain

sufficiently high to be saturated using the minimum drive

current (typically 20mA). A 100

Ω ~ 200Ω resistor is

recommended to be inserted between the 5VDLSB pin

and Base node of the PNP transistor for limiting the base

current.

Q3, Q5

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 (S0, S1)

state. Similar RDS(ON) criteria apply in these cases as well,

unlike the PMOS, however, these NMOS transistors get

the benefit of an increased VGS drive (approximately 8V

and 7V respectively).

The NPN transistor used as sleep-state pass element on

the 3.3VDUAL output must have a minimum current gain of

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

circuit operating temperature range.

Transistor Selection/Considerations

RT9641BGS Datasheet(PDF) 10 Page - Richtek Technology Corporation

RT9641BGS Datasheet(HTML) 10 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11

Datasheet Download

Link URL