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US1075
7.5A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR FEATURES
1V Dropout at Full Load Current Fast Transient Response 1% Voltage Reference Initial Accuracy Output Current Limiting Built-in Thermal Shutdown
DESCRIPTION
The US1075 product is a low dropout three terminal adjustable regulator with minimum of 7.5A output current capability. This product is specifically designed to provide well regulated supply for low voltage IC applications such as PentiumTM P54CTM,P55CTM as well as GTL+ termination for Pentium ProTM and KlamathTM processor applications . The US1075 is also well suited for other processors such as CyrixTM,AMD and Power PCTMapplications. The US 1075 is guaranteed to have <1.2V drop out at full load current making it ideal to provide well regulated outputs such as 3.3V with input supply voltage as low as 4.5V minimum.
APPLICATIONS
Low Voltage Processor Applications such as : P54CTM,P55CTM,Cyrix M2TM, POWER PCTM,AMD GTL+ Termination PENTIUM PROTM, KLAMATHTM Low Voltage Memory Termination Applications Standard 3.3V Chip-Set and Logic Applications
TYPICAL APPLICATION
5V
C1 1500uF Vin
3
US1075
Vout
2
R1 121
3.3V
C2 2x 1500uF
Adj
1075app1-1.0
1
R2 200
Typical Application of US1075 in a 5V to 3.3V regulator
Notes: Pentium P54C,P55C ,Klamath,Pentium Pro,VRE,are trade marks of Intel Corp.Cyrix M2 is trade mark of Cyrix Corp. Power PC is trade mark of IBM Corp.
PACKAGE ORDER INFORMATION
Tj (C) 0 TO 150 3 PIN PLASTIC TO220 (T) US1075CT 3 PIN PLASTIC TO263 (M) US1075CM 3 PIN PLASTIC POWER FLEX (P) US1075CP
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US1075
ABSOLUTE MAXIMUM RATINGS
Input Voltage (Vin) .................................................................. 7V Power Dissipation ............................................ Internally Limited Storage Temperature Range .............................. -65C TO 150C Operating Junction Temperature Range .................. 0C TO 150C
PACKAGE INFORMATION
3 PIN PLASTIC TO220 ( T )
FRONT VIEW 3
3 PIN PLASTIC TO263 ( M )
FRONT VIEW
3 PIN PLASTIC POWER FLEX ( P )
FRONT VIEW
Vin Vout Adj
3
Vin Vout Adj Tab is Vout
3
Vin Vout Adj
Tab is Vout
2
Tab is Vout
2
2
1
1
1
JT=2.7C/W JA=60C/W
JA=35C/W for 1" Square pad
JA=35C/W for 1" Square pad
ELECTRICAL SPECIFICATIONS
Unless otherwise specified ,these specifications apply over ,Cin=1uF,Cout=10uF,and Tj=0 to 150C.Typical values refer to Tj=25C. PARAMETER Reference Voltage Line Regulation Load Regulation (note 1) Dropout Voltage (note 2) Current Limit Minimum Load Current (note 3) Thermal Regulation Ripple Rejection Adjust Pin Current Adjust Pin Current Change Temperature Stability Long Term Stability RMS Output Noise SYM VREF TEST CONDITION MIN TYP Io=10mA,Tj=25C,(Vin-Vo)=1.5V 1.238 1.250 Io=10mA, (Vin-Vo)=1.5V 1.225 1.250 Io=10mA,1.3V<(Vin-Vo)<7V Vin=3.3V,Vadj=0,10mAVO
IADJ
Note 1 : Low duty cycle pulse testing with Kelvin connections are required in order to maintain accurate data. Note 2 : Drop-out voltage is defined as the minimum differential voltage between Vin and Vout required to maintain regulation at Vout. It is measured when the output voltage drops 1% below its nominal value.
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.
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PIN DESCRIPTIONS
PIN # 1 2 3 PIN SYMBOL Adj Vout Vin PIN DESCRIPTION A resistor divider from this pin to the Vout pin and ground sets the output voltage. The output of the regulator. A minimum of 10uF capacitor must be connected from this pin to ground to insure stability. The 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 1.3V higher than Vout in order for the device to regulate properly.
BLOCK DIAGRAM
Vin 3 2 Vout
+ +
1.25V CURRENT LIMIT
THERMAL SHUTDOWN
1075blk1-1.0
1 Adj
Figure 1 - Simplified block diagram of the US1050
APPLICATION INFORMATION
Introduction
The US1075 adjustable Low Dropout (LDO) regulator is a 3 terminal device which can easily be programmed with the addition of two external resistors to any voltages within the range of 1.25 to 5.5 V.This regulator unlike the first generation of the 3T regulators such as LM117 that required 3V differential between the input and the regulated output,only needs 1.3V differential to maintain output regulation. This is a key requirement for today's microprocessors that need typically 3.3V supply and are often generated from the 5V supply. Another major requirement of these microprocessors such as the Intel P54CTM is the need to switch the load current from zero to several amps in tens of nanoseconds at the processor pins ,which translates to an approximately 300 to 500 nS current step at the regulator . In addition, the output voltage tolerances are also extremely tight and they include the transient response as part of the specification.For example Intel VRETM specification calls for a total of 100mV including initial tolerance,load regulation and 0 to 4.6A load step. The US1075 is specifically designed to meet the fast current transient needs as well as providing an accurate initial voltage , reducing the overall system cost with the need for fewer output capacitors.
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US1075
Output Voltage Setting
The US1075 can be programmed to any voltages in the range of 1.25V to 5.5V with the addition of R1 and R2 external resistors according to the following formula: R2 VOUT = VREF 1 + + IADJ x R2 R1 Where : VREF = 125 V Typically . IADJ = 50 uA Typically R1 & R2 as shown in figure 2 regulation is achieved when the bottom side of R2 is connected to the load and the top side of R1 resistor is connected directly to the case or the Vout pin of the regulator and not to the load. In fact , if R1 is connected to the load side, the effective resistance between the regulator and the load is gained up by the factor of (1+R2/ R1) ,or the effective resistance will be ,Rp(eff)=Rp*(1+R2/ R1).It is important to note that for high current applications, this can represent a significant percentage of the overall load regulation and one must keep the path from the regulator to the load as short as possible to minimize this effect.
Vin
Vin
Vout
Vout
Vin
Vref R1 Adj Vin Vout
PARASITIC LINE RESISTANCE
US1075
Adj
Rp
US1075
R1 RL
IAdj = 50uA
R2
1075app2-1.0
R2
Figure 2 - Typical application of the US1075 for programming the output voltage. The US1075 keeps a constant 1.25V between the output pin and the adjust pin. By placing a resistor R1 across these two pins a constant current flows through R1, adding to the Iadj current and into the R2 resistor producing a voltage equal to the (1.25/R1)*R2 + Iadj*R2 which will be added to the 1.25V to set the output voltage. This is summarized in the above equation. Since the minimum load current requirement of the US1075 is 10 mA , R1 is typically selected to be 121 resistor so that it automatically satisfies the minimum current requirement. Notice that since Iadj is typically in the range of 50uA it only adds a small error to the output voltage and should only be considered when a very precise output voltage setting is required. For example, in a typical 3.3V application where R1=121 and R2=200 the error due to Iadj is only 0.3% of the nominal set point.
1075app3-1.0
Figure 3 - Schematic showing connection for best load regulation
Stability
The US1075 requires the use of an output capacitor as part of the frequency compensation in order to make the regulator stable. Typical designs for microprocessor applications use standard electrolytic capacitors with a typical ESR in the range of 50 to 100 m and an output capacitance of 500 to 1000uF. Fortunately as the capacitance increases, the ESR decreases resulting in a fixed RC time constant. The US1075 takes advantage of this phenomena in making the overall regulator loop stable.For most applications a minimum of 100uF aluminum electrolytic capacitor such as Sanyo MVGX series ,Panasonic FA series as well as the Nichicon PL series insures both stability and good transient response.
Load Regulation
Since the US1075 is only a 3 terminal device , it is not possible to provide true remote sensing of the output voltage at the load.Figure 3 shows that the best load
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Thermal Design
The US1075 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 selected heat sink be chosen such that during maximum continuous load operation the junction temperature is kept below this number. The example below shows the steps in selecting the proper Regulator heat sink for the worst case current consumption using Intel 200MHz microprocessor as the load . Assuming the following specifications : VIN =5 V VO =3.5 V IOUTMAX =4.6 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 =4.6 x(5 - 3.5) =6.9 W 2) Select a package from the Regulator data sheet 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=0.05C/W (Heat sink to Case thermal resistance for Black Anodized) TS = TJ-PD x (JC + CS) TS =135- 6.9 x(2.7 +0.05) =116 C 4) With the maximum heat sink temperature calculated in the previous step, the Heat Sink to Air thermal resistance (sa) is calculated by first calculating the temperature rise above the ambient as follows : T = TS - TA =116 -35=81 C PD = IO UT x( VIN- VO UT ) T=Temperature Rise Above Ambient T PD 81 . SA = =117 C / W 6.9 SA = 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 select a heat sink that results in lower temperature rise than the one calculated in previous step. The following heat sinks from AAVID and Thermaloy meet this criteria.
Thermalloy AAVID
Air Flow (LFM) 0 100 200 300 6021PB 6021PB 6073PB 6109PB 534202B 534202B 507302 575002
400 7141D 576802B
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