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| US1261 DUAL 6A AND 1A LOW DROPOUT POSITIVE FIXED OUTPUT REGULATOR FEATURES Guaranteed to provide 1.5V and 2.5V Supplies with 3.1V input. Fast Transient Response 1% Voltage Reference Initial Accuracy Built in Thermal Shutdown DESCRIPTION The US1261 product using a proprietary process combines a dual low dropout regulators with fixed outputs of 1.5V and 2.5V in a single package with the 1.5V output having a minimum of 6A and the 2.5V having a 1A output current capability. This product is specifically designed to provide well regulated supplies from 3.3V to generate 1.5V for GTL+ termination resistor supply and 2.5V clock supply for the new generation of the Pentium II processor applications. APPLICATIONS Pentium II Processor Applications TYPICAL APPLICATION 3.3V C1 Vout1 Vin 5 4 3 2 1 C4 C2 C3 2.5V / 1A US1261 Gnd Vout2 Vctrl 1.5V / 6A 1261app1-1.3 5V R1 Typical application of US1261 in a Pentium II processor application PACKAGE ORDER INFORMATION Tj (C) 0 TO 150 5 PIN PLASTIC TO220(T) US1261CT 5 PIN PLASTIC TO263(M) US1261CM 5 PIN PLASTIC POWER FLEX(P) US1261CP Rev. 1.7 3/22/99 3-1 US1261 ABSOLUTE MAXIMUM RATINGS Input Voltage (Vin) ............................................................. 10V Power Dissipation ............................................ Internally Limited Storage Temperature Range ................................ -65C TO 150C Operating Junction Temperature Range ...................... 0C TO 150C PACKAGE INFORMATION 7 PIN PLASTIC TO220 FRONT VIEW 5 4 3 2 1 7 PIN PLASTIC TO263 FRONT VIEW 5 4 3 2 1 7 PIN POWER FLEX (P) FRONT VIEW 5 4 3 2 1 Vout1 Vin Gnd Vout2 Vctrl Vout1 Vin Gnd Vout2 Vctrl Vout1 Vin Gnd Vout2 Vctrl JT =2.7C/W JA =60C/W JA =30C/W for 1"sq pad JA =30C/W for 1"sq pad ELECTRICAL SPECIFICATIONS Unless otherwise specified ,these specifications apply over ,Cin=1 uF,Cout=100uF,and Tj=0 to 150C.Typical values refer to Tj=25C. Ifl=6A for output #2 & 1A for output #1.Vctrl=5V,Vin=3.3V. PARAMETER SYM TEST CONDITION MIN TYP MAX UNITS Vctrl Input Voltage 3.0 V Output Voltage #2 Vo2 Io=10mA, Tj=25C 1.485 1.500 1.515 V Io=10mA 1.470 1.500 1.530 Output Voltage #1 Vo1 Io=10mA, Tj=25C 2.462 2.500 2.537 V Io=10mA 2.425 2.500 2.575 Line Regulation Io=10mA, 3.1V Note 3 : Minimum load current is defined as the minimum current required at the output in order for the output voltage to maintain regulation. Typically the resistor dividers are selected such that it automatically maintains this current. 3-2 Rev. 1.7 3/22/99 US1261 PIN DESCRIPTIONS PIN # 2 5 4 PIN SYMBOL Vout2 Vout1 Vin PIN DESCRIPTION The output #2 (high current) of the regulator. A minimum of 100uF capacitor must be connected from this pin to ground to insure stability. The output #1 (low current) of the regulator. A minimum of 100uF capacitor must be connected from this pin to ground to insure stability. The power input pin of the regulator. Typically a large storage capacitor is connected from this pin to ground to insure that the input voltage does not sag below the minimum drop out voltage during the load transient response. This pin must always be higher than both Vout pins by the amount of the dropout voltage(see datasheet) in order for the device to regulate properly. This pin is connected to GND. It is also the TAB of the package. The control input pin of the regulator. This pin via a 10 resistor is connected to the 5V supply to provide the base current for the pass transistor of both regulators. This allows the regulator to have very low dropout voltage which allows one to generate a well regulated 2.5V supply from the 3.3V input. A high frequency, 1 uF capacitor is connected between this pin and Vin pin to insure stability. 3 1 Gnd Vctrl BLOCK DIAGRAM Vin 4 Vctrl 1 5 Vout1 THERMAL SHUTDOWN 1.20V + 3 Gnd 1261blk1-1.1 2 Vout2 Figure 1 - Simplified block diagram of the US1261 Rev. 1.7 3/22/99 3-3 US1261 APPLICATION INFORMATION Introduction The US1261 is a dual fixed output Low Dropout (LDO) regulator available in a 5 pin TO-220 or TO-263 packages. This voltage regulator is designed specifically for PentiumII processor applications requiring 2.5V and 1.5 V supplies, eliminating the need for a second regulator resulting in lower overall system cost. The US1261 is designed to take advantage of 5V supply to provide the drive for the pass transistor, allowing 2.5V supply to be generated from 3.3V input.This feature improves the power dissipation of the 2.5V regulator substantially allowing a smaller heat sink to be used for the application. Compared to the US1260 dual adjustable regulator, the US1261 includes the resistor dividers that are otherwise needed with the US1260, eliminating 4 external components and their tolerances, resulting in a more accurate initial accuracy for each output voltage. Other features of the device include; fast response to sudden load current changes, such as GTL+ termination application and thermal shutdown protection to protect the device if an overload condition occurs. Stability The US1261 requires the use of an output capacitor as part of the frequency compensation in order to make the regulator stable. Typical designs for the microprocessor applications use standard electrolytic capacitors with typical ESR in the range of 50 to 100 m and the output capacitance of 500 to 1000uF. Fortunately as the capacitance increases, the ESR decreases resulting in a fixed RC time constant. The US1261 takes advantage of this phenomena in making the overall regulator loop stable. For most applications a minimum of 100uF aluminum electrolytic capacitor with the maximum ESR of 0.3 such as Sanyo, MVGX series ,Panasonic FA series as well as the Nichicon PL series insures both stability and good transient response. The US1261 also requires a 1 uF ceramic capacitor connected from Vin to Vctrl and a 10, 0.1W resistor in series with Vctrl pin in order to further insure stability. Thermal Design The US1261 incorporates an internal thermal shutdown that protects the device when the junction temperature exceeds the maximum allowable junction temperature. Although this device can operate with junction temperatures in the range of 150C ,it is recommended that the SA selected heat sink be chosen such that during maximum continuous load operation the junction temperature is kept below this number. Two examples are given which shows the steps in selecting the proper regulator heat sink for driving the Pentium II processor GTL+ termination resistors and the Clock IC using 1261 in TO220 and TO-263 packages. Example # 1 Assuming the following specifications : VIN = 3.3V VOUT 2 = 1.5 V VOUT 1 = 2.5 V IOUT 2 MAX = 5.4A IOUT 1 MAX = 0.4 A TA = 35 C The steps for selecting a proper heat sink to keep the junction temperature below 135C is given as : 1) Calculate the maximum power dissipation using : PD = IOUT1 x ( VIN - VOUT1) + IOUT2 x ( VIN - VOUT2) PD = 0.4 x ( 3.3 - 2.5 ) + 5.4 x ( 3.3 - 1.5) = 10 W 2) Select a package from the datasheet and record its junction to case (or Tab) thermal resistance. Selecting TO220 package gives us : JC =2.7 C / W 3) Assuming that the heat sink is Black Anodized, calculate the maximum Heat sink temperature allowed : Assume , CS = 0.05 C/W (Heat sink to Case thermal resistance for Black Anodized) TS = TJ - PD x (JC + CS) TS = 135 - 10 x ( 2.7 + 0.05) = 107.4 C 4) With the maximum heat sink temperature calculated in the previous step, the Heat Sink to Air thermal resistance SA is calculated as follows : T = TS - TA = 107.4 - 35 = 72.4 C SA = T PD 72.4 = = 7.24 C / W 10 5) Next , a heat sink with lower SA than the one calculated in step 4 must be selected. One way to do this is to simply look at the graphs of the "Heat Sink Temp Rise Above the Ambient" vs. the "Power Dissipation" and Rev. 1.7 3/22/99 3-4 US1261 select a heat sink that results in lower temperature rise Layout Consideration than the one calculated in previous step. The following heat sinks from AAVID and Thermaloy meet this crite- The US1261 like all other high speed linear regulators need ria. to be properly laid out to insure stable operation. The most important component is the output capacitor, which Air Flow (LFM) 0 100 200 300 400 needs to be placed close to the output pin and conThermalloy 7021B 7020B 6021PB 7173D 7141D nected to this pin using a plane connection with a AAVID 593101B 551002B 534202B 577102B 576802B low inductance path. Note : For further information regarding the above companies and their latest product offering and application support contact your local representative or the numbers listed below: Thermalloy AAVID PH# (214) 243-4321 PH# (603) 528-3400 Example # 2 : Assuming the following specifications : VIN = 3.3V VOUT 2 = 1.5 V VOUT 1 = 2.5 V IOUT 2 MAX = 1.5A IOUT 1 MAX = 0.2 A TA = 35 C The steps for selecting a proper heat sink to keep the junction temperature below 135C is given as : 1) Calculate the maximum power dissipation using : PD = IOUT1 x ( VIN - VOUT1) + IOUT2 x ( VIN - VOUT2) PD = 0.2 x ( 3.3 - 2.5) + 1.5 x (3.3 - 1.5) = 2.86 W 2) Assuming a TO-263 surface mount package, the junction to ambient thermal resistance of the package is: JA = 30 C / W for 1" square pad area 3) The maximum junction temperature of the device is calculated using the equation below : TJ = TA + PD x JA TJ = 35 + 2.86 x 30 = 121 C Since this is lower than our selected 135C maximum junction temperature (150C is the thermal shutdown of the device), TO-263 package is a suitable package for our application. Rev. 1.7 3/22/99 3-5 US1261 TYPICAL APPLICATION PENTIUM TM APPLICATION 3.3V C1 Vout1 Vin 5 4 3 2 1 C4 C2 C3 2.5V / 1A US1261 Gnd Vout2 Vctrl 1.5V / 6A 1261app1-1.3 5V R1 Figure 4 - Typical application of US1261 in the Pentium TM design with the 1.5V output providing for GTL+ termination while 2.5V supplies the clock chip. Notes: Pentium is trade mark of Intel Corp. Ref Desig U1 C1,C4 C3 C2 R1 HS1 Description Qty Part # Manuf Dual LDO Regulator 1 US1261CM Unisem Capacitor 2 Elect,680uF,EEUFA1A681L Panasonic Capacitor 1 Elect,220uF,6.3V,ECAOJFQ221 Panasonic Capacitor 1 Ceramic,1uF,16V,Z5U Resistor 1 3, 0.1W , 0805 SMT Panasonic Heat Sink 1)Use 1" Square Copper Pad area if Iout2<1.7A & Iout1<0.2A. 2)For Iout2<3A & Iout1<0.5A Use US1261CT and Thermalloy 6030B 3)For Iout2<5.4A & Iout1<0.5A Use US1261CT and Thermalloy 7021B 3-6 Rev. 1.7 3/22/99 |
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