Design Considerations 3.3V Pass Regulators Wolbert General Descri

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Design Considerations 3.3V Pass Regulators Wolbert General Descri

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Design Considerations 3.3V Pass Regulators Wolbert
General Description
rise 3.3V logic memory components personal computer systems created demand 3.3V power supplies. Several options exist computer system designer. these options provide both 3.3V 5.0V from main system power supply switch matrix voltage selection (see Application Hint representative circuitry). drawbacks this technique exist: extra 3.3V output costs money; (2), current levels above about MOSFETs used 3.3V portion matrix require exceptionally resistance maintain output tolerance quite expensive. Another option uses existing high current supply employs drop-out (LDO) linear regulator provide 3.3V. This cost option, requiring only short design work little motherboard space. Linear regulators provide clean, accurate output radiate RFI, government certification jeopardized. They fast starting, provide ON/OFF control error flag that indicates power system trouble. current levels, thermal considerations difficult; however, currents amperes, resulting heat troublesome. This note discusses option, including choosing between simple three terminal regulators full-featured five terminal regulators, provides formulas, calculations, selection commercial heat sinks powering 3.3V logic circuitry requiring amps from standard supply. Additionally, "trick" reducing heat sink requirements distributing power dissipation with series resistor discussed.
MIC29150 family 3.3V CMOS compatible. When this input pulled above approximately 1.4V, regulator activated. special feature this regulator family zero power consumption when inactive. Whenever digital control input low, internal circuitry biased OFF. tiny leakage current, measured nanoamperes, flow). Three terminal regulators used whenever ON/OFF control necessary processing power available flag output information. Three terminal regulators need only single output filter capacitor design effort minimal. Five terminal regulators provide functionality three devices PLUS allow power supply quality monitoring ON/OFF switching "sleep" mode applications.
Thermal Design Considerations
Micrel drop-out (LDO) regulators very easy use. Only external filter capacitor necessary operation electrical design effort minimal. many cases, thermal design also quite simple, drop-out characteristic Micrel's LDOs. Unlike other linear regulators, Micrel's LDOs operate with drop-out voltages 300mV-often less. resulting Voltage Current power loss quite small with moderate output current. higher currents, however, selecting correct heat sink important chore. Power dissipation linear regulator [(VIN VOUT) IOUT] (VIN IGND) Where: Power dissipation Input voltage applied regulator VOUT Regulator output voltage IOUT Regulator output current IGND Regulator biasing currents Proper design dictates worst case values parameters. Worst case high supply; this case, 5.25V. Worst case VOUT thermal considerations minimum, 3.3V 3.234V.1 IOUT taken highest steady-state value. ground current value comes from device's datasheet, from graph IGND IOUT. Armed with this information, calculate thermal resistance (SA) required heat sink using following formula: ---------
Choose Five Terminal Regulators?
What extra pins five linear regulators provide? After all, three terminal regulators give Input, Output, Ground; what else necessary? Five terminal devices allow system designer monitor power quality load digitally switch supply OFF. Power quality monitored flag output. When output voltage within percent desired value, flag high, indicating "Good". output drops, because either input voltage regulator over-current condition, flag drops signal fault condition. controller monitor this output make decisions regarding system's readiness. example, initial power-up, flag will instantaneously read high pulled external supply), soon input supply regulator reaches about flag pulls low. stays until regulator output nears desired value. With MIC29150 family drop-out linear regulators, flag rises when output voltage reaches about desired value. 3.3V system, flag indicates "power good" with VOUT 3.2V. Digital power control allows "sleep" mode operation results better energy efficiency. ENABLE input 1997
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Assuming Micrel with maximum temperature 125°C TO-220 package with 2°C/W mounting resistance (CS) 1°C/W2, operating ambient temperature 50°C, 50°C ---------- 1°C/W) °C/W 10.5W Performing similar calculations 1.25A, 1.5A, 2.0A, 2.5A, 3.0A, 4.0A, 5.0A gives results shown Table Regulator MIC29150 MIC29150 MIC29300 MIC29300 MIC29300 MIC29500 MIC29500 IOUT 1.25A 1.5A 2.0A 2.5A 3.0A 4.0A 5.0A 10.5 SA(°C/W)
fortunate enough have some forced airflow, reductions heat sink cost space possible characterizing speed-even slow airstream significantly assists cooling. with natural convection, small allowing airstream pass necessary. Fins should located maximize airflow along them. Orientation with respect vertical important, airflow dominates.
Output 1.5A
Ambient Temperature 40°C 50°C 60°C 24°C/W 21°C/W 17°C/W 5.1°C/W 4.1°C/W 3.2°C/W
Table Ambient temperature affects heat sink requirements example, will select heat sinks 1.5A outputs. consider four airflow cases: natural convection, feet/minute (1m/sec), feet/minute (1.5m/sec), feet/minute (2m/sec). Table shows heat sinks these velocities; note rapid reduction size weight (fin thickness) when forced available. Consulting manufacturer's charts,3,4 variety sinks made that suitable application. (10.5W worst case package dissipation) natural convection, sinks sizable, 1.5A (3.2W worst case package dissipation) feet/minute airflow, modest heat sinks adequate. heat sink required applications still massive expensive. There better manage heat problems: take advantage very drop voltage characteristic Micrel's Super PNPregulators dissipate some power externally series resistance.5 distributing voltage drop between this cost resistor regulator, distribute heating, reduce size regulator heat sink. Knowing worst case voltages system peak current requirements, select resistor that drops portion excess voltage without sacrificing performance. maximum value resistor calculated from: (VOUT VDO) RMAX ---------------------- IOUT PEAK IGND Where: supply 4.75V) maximum output voltage across full temperature range (3.3V 3.366V) worst case dropout voltage across full temperature range (600mV) IOUT PEAK maximum 3.3V load current IGND regulator ground current.
Table Micrel power dissipation heat sink requirements various 3.3V current levels. Table shows effect maximum ambient temperature heat sink thermal properties. Lower thermal resistances require physically larger heat sinks. table clearly shows cooler running systems need smaller heat sinks, common sense suggests.
Heat Sink Selection
With this information specify heat sink. worst case still (natural convection). heat sink should mounted that least 0.25 inches (about 6mm) separation exists between sides sink other components system case. Thermal properties maximized when heat sink mounted that natural vertical motion warm directed along long axis sink fins.
MIC29500-3.3
3.3V 47µF
Figure Using Micrel very simple. Only output filter capacitor necessary. Here, 3.3V produced from nominal input.
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output example:
Airflow Output Current 1.5A
Thermalloy 6049PB
Thermalloy 6232 Thermalloy 6034 Thermalloy 6391B AAVID 504222B AAVID 563202B AAVID 593202B AAVID 534302B Thermalloy 7021B Thermalloy 6032 Thermalloy 6234B
4.75 (3.366 0.6) 0.784V RMAX ------------------ -------- 0.154 0.08 5.08A power drop across this resistor (IOUT PEAK IGND)2 4.0W. This subtracts directly from 10.5W regulator power dissipation that occurs without resistor, reducing regulator heat generation 6.5W. PD(Regulator) PD(R Considering resistor tolerances standard values leads 0.15 resistor. This produces nominal power savings 3.9W. With worst-case tolerances, regulator power dissipation drops 6.8W maximum. This heat drop reduces heat sinking requirements MIC29500 significantly. smaller heat sink with larger thermal resistance. Now,
ft./min. (2m/sec)
ft./min. (1.5m/sec)
ft./min. (1m/sec) AAVID 577002 Thermalloy 6043PB Thermalloy 6045B
AAVID 508122 AAVID 552022 AAVID 533302 Thermalloy 7025B Thermalloy 7024B Thermalloy 7022B Thermalloy 6101B
Natural Convection forced airflow)
AAVID 576000 AAVID 533602B (vertical) AAVID 574802 AAVID 519922B (horizontal) 592502 AAVID 532802B (vertical) 579302 Thermalloy 6299B (vertical) Thermalloy 6238B Thermalloy 7023 (horizontal) Thermalloy 6038 Thermalloy 7038
Table Commercial heat sinks 1.5A 5.0A applications
Airflow
ft./min. (2m/sec)
Heat Sink Model
AAVID 530700 AAVID 574802 Thermalloy 6110 Thermalloy 7137, 7140 Thermalloy 7128 AAVID 57302 AAVID 530600 AAVID 577202 AAVID 576802 Thermalloy 6025 Thermalloy 6109 Thermalloy 6022 AAVID 575102 AAVID 574902 AAVID 523002 AAVID 504102 Thermalloy 6225 Thermalloy 6070 Thermalloy 6030 Thermalloy 6230 Thermalloy 6021, 6221 Thermalloy 7136, 7138 AAVID 563202 AAVID 593202 AAVID 534302 Thermalloy 6232 Thermalloy 6032 Thermalloy 6034 Thermalloy 6234
ft./min. (1.5m/sec)
ft./min. (1m/sec)
heat sink with 8.3°C/W thermal characteristics suitable- nearly factor better than without resistor. Table lists representative heat sinks meeting these conditions. 1.5A output application using MIC29150, calculate maximum 0.512. Using 0.51, savings least 1.1W achieved, dropping power dissipation only 2.0W-a heat sink probably required. This circuit shown Figure Another option exists designers lower current systems. MIC29150 MIC29300 regulators available surface mount derivative TO-220 package, TO263, which soldered directly board. separate heat sink necessary, copper area board acts heat exchanger. further information, refer Micrel's Application Hint "P.C. Board Heat Sinking".
Natural Convection forced airflow)
Table Representative commercial heat sinks 5.0A output example using series dropping resistor. Assumptions: 50°C, 0.15 ±5%, IOUT 5.0A, 2°C/W, 1°C/W, resulting required 8.0°C/W. 1997 3-181
MIC29501 worst case power dissipation: 6.5W
0.15,
MIC29501-3.3
Control
3.3V Flag 47µF
0.8V
Figure Producing 3.3V with minimal heat sink requirements. 0.15 resistor dissipates excess power, reducing regulator heat generation. resistor needs heat sink.
MIC29151 worst case power dissipation:
0.51,
MIC29151-3.3
Control
3.3V 1.5A Flag 22µF
0.8V
Figure MIC29151-3.3 produces 1.5A 3.3V. heat sink necessary most situations when external power sharing resistor employed.
Notes
NOTE MIC29150, MIC29300, MIC29500, MIC29750 regulator family feature trimmed outputs guaranteed under standard conditions. Across full temperature range, with load input voltage variations, device output voltage varies less than worst case. NOTE mounting MIC29150 family regulators grounded. estimated value assumes electrical insulation between mounting heat sink. NOTE AAVID Engineering, Inc., Kool Path, Laconia, 03247. (603) 528-3400. NOTE Thermalloy Inc., P.O. 810839, Dallas, 75381. (214) 243-4321. NOTE Super PNPis registered trademark Micrel, Inc.
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Design Considerations 3.3V Pass Regulators Wolbert General Descri

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