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RT9641APS Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT9641APS Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 11 page  RT9641A/B 10 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. Q1 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. Q4 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). Q2 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 |
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