Application Note 58 September 1993 5V to 3.3V Converters for Microprocessor Systems Robert Dobkin, Mitchell Lee, Dennis O'Neill and Milt Wilcox Introduction Linear Regulators The new generation of high performance microprocessors are built on dense, low breakdown voltage processes in order to accommodate increased transistor counts. These new processors require high current power at 3.3V, developed from the 5V input used to power the rest of the system. Special techniques are required to ensure proper operation of the microprocessor and good heat dissipation within the computer system. Table 1 shows the range of components available for linear regulation of 3.3V with a 5V input. With only 1.7V of headroom, low dropout is essential. Low dropout regulators are available delivering currents from 125mA to 7.5A, allowing almost any microprocessor to be powered with a local 3.3V generation circuit. The first four devices (LT1020, LT1120, LT1121 and LT1129) are PNP micropower low dropout regulators. Since PNP transistors are much larger than monolithic NPNs, higher current regulators use an NPN pass device. The LT1117, LT1086, LT1083 through LT1085, and LT1087 all use NPN pass devices. The NPN structure requires about 1.2V headroom compared to the 400mV to 500mV dropout typical of PNP regulators, and ground current of 5mA or 10mA, independent of output current. Because of this constant quiescent current, the LT1117 and LT1083 family are not suitable for applications requiring micropower standby. The 3.3V supply may be either a linear or switching type. For most applications a linear regulator is preferable since it minimizes components and has acceptable efficiency for a whole computer system. In portable computers where high efficiency is paramount because of battery operation, a switching supply is necessary. This application note contains a collection of 3.3V regulator circuits, each optimized for a 5V input and surface mount technology. The circuits are split into two categories, linear and switching, and further arranged by current capability. Most of the circuits, with the possible exception of the high current linear regulators, are surface mountable. Where appropriate, part numbers are given for surface mount coils, capacitors, and diodes. Resistors and small capacitors, unless there are special characteristics, are generic and manufacturer’s part numbers are not shown. Both linear and switching regulators are available for the purpose of converting 5V to 3.3V. In general, the linear regulators are the best choice at lower (≤ 3A) current levels where their dissipation is minimized, or in lineoperated equipment where 66% theoretical efficiency is acceptable. Switchers are favored in higher current and efficiency-conscious applications. Efficiencies in the 90% to 95% range are the norm for switchers described in this application note. Table 1. 3.3V Linear Regulators LOAD CURRENT DEVICE PASS DEVICE SHUTDOWN CURRENT TOLERANCE* 125mA LT1020 PNP 40µA 2.1% (69mV) 125mA LT1120 PNP 40µA 2.1% (69mV) 150mA LT1121-3.3 PNP 16µA 3% (100mV) 700mA LT1129-3.3 PNP 16µA 3% (100mV) 800mA LT1117-3.3 NPN – 2% (65mV) 1.5A LT1086-3.3 NPN – 1.6% (53mV) 3A LT1085-3.3 NPN – 1.6% (53mV) 5A LT1084 NPN – 1.9% (61mV) 7.5A LT1083 NPN – 1.9% (61mV) 10A 2 × LT1087 NPN – 1.9% (61mV) *Includes line, load, and temperature variations. Adjustable parts also include worst case effect of external 1% resistors. AN58-1 Application Note 58 Controlling Transient Loads Microprocessors require the input voltage to be maintained within ±5% under worst case transients. The dynamic nodes internal to the processor are sensitive to voltage. Transients drawn by the processor are so fast that no active loop can respond in time. Adequate reserves of charge must be maintained in a group of capacitors to supply this current until the regulator can respond. This is true for both linear and switching regulators. Modern power saving processors may draw large transient currents unlike older processors. Many include sleep modes which slow down or stop the processor when it is not in use. The transition from normal operation to sleep mode, or sleep mode to normal operation usually causes a large step in power supply current. The supply current can jump several amps in a matter of nanoseconds—far faster than any regulator can respond. Proper printed circuit layout and bypass capacitors are needed to provide these current transients. Typical printed circuit board layouts include a power plane and a ground plane which are separate from the rest of the system. Connected to the pins of the processor are small 100nF bypass capacitors, as is common practice in processor layout. These capacitors control the voltage for very fast transients in the 10ns to 100ns time period. Further from the processor is a large reservoir capacitor located at the output of the regulator. This capacitor is typically 100µF to 200µF and provides the energy reservoir for 100ns to 2µs until the control loop in the regulator can correct the output. For longer durations, the control loop in the regulator keeps the output voltage constant. When the load is released the overshoot must be controlled as well, and the capacitors absorb the energy and limit overshoot from the regulator. The capacitors must be low inductance and connected directly to the power plane close to the processor. Several inches of trace going to a capacitor can be sufficient to cause large transient voltages under changing load conditions because of the inductance in the circuit traces. The power cycling associated with the new processors makes this situation far worse than older processors which operated continuously. An input bypass capacitor is placed close to the regulator to provide a low source impedance. The input to the AN58-2 regulator must also provide an energy reservoir. Typically, here again, is a 10µF to 100µF capacitor that provides the energy at the input of the regulator during a load transient. This capacitor is mandatory since the regulator is usually situated far from the input power supply. Both the input and output capacitors play a role in the stability of the regulator and help assure adequate transient response. The capacitor values shown in this app note represent the minimum required for stability. Additional capacitance may be necessary to handle load transients. See the design example. Thermal Design Heat sinking is an important consideration. For processors drawing 5A, the power dissipated in the regulator can be as high as 8.5W. This amount of power requires adequate heat sinking internal to the computer system. The general rule for surface mounted components is to maximize the amount of copper connected to the leads of the IC. Flood all open areas, intermediate layers, and the back side of the board with copper. This aids in spreading and radiating the heat. Surface mount components can dissipate up to 2.5W (1.5A output current) using only circuit traces and ground planes totaling 2 or 3 square inches. For higher output currents a larger heat sink is needed. To compute the thermal resistance of the heat sink it is necessary to know the maximum operating temperature of the regulator, maximum ambient inside the computer enclosure and the air flow over the heat sink. For a power dissipation of 5W (3A output current), maximum junction temperature of 125°C and maximum ambient temperature of 80°C: θHS = (TJ – TA)/PD – θJC θHS = (125 – 80)/5 – 3 θHS = 6°C/W where: PD = Power Dissipation (°C) TJ = Maximum Junction Temperature (°C) TA = Maximum Ambient Temperature (°C) θJC = Junction to Case Thermal Resistance of IC (°C/W) θHS = Heat Sink Thermal Resistance (°C/W) Application Note 58 The heat sink for this application must have a thermal resistance of 6°C/W or less. Figure 1 shows the effect of air flow over the surface of a 6°C/W heat sink (Thermalloy 7025B-MT). With no air flow the thermal resistance is dominated by convection currents; this is why the graph stops at approximately 100 feet per minute air flow. A much smaller heat sink could be used in this application if some air flow, such as from the computer’s cooling fan, could be guaranteed. At higher output currents and dissipations it is almost always necessary to provide some air flow in order to avoid an unreasonably large heat sink. Linear Technology regulators in the LT1083/4/5/6, LT1117 and LT1121/9 families are designed to withstand over 5V with no problems. The MOSFET has an on resistance of approximately 30mΩ when the gate is driven to 12V. There is a 12V power supply available in most systems and a high value resistor can be used to tie the gate of the MOSFET high. If 12V is not available, an LTC1157 high-side driver (Figure 3) can be used to drive the gate of the MOSFET. 5V INPUT 6 6 THERMAL RESISTANCE (°C/W) 4 5 5 SHORT FOR 3.3V OPEN FOR 5V (SHORTING LINK IN µP PACKAGE) 4 3 2 LTC1157 3 5V INPUT 7 MTD3055EL (1.5A) MTB30N06EL (3A) MTB50N06EL (5A) LINEAR REGULATOR 3.3V OR 5V OUTPUT 2 AN58 • F03 Figure 3. LT1157 Switches Between 5V and 3.3V 1 100 1000 AIR VELOCITY (FEET PER MINUTE) Switching Regulators AN58 • F01 Figure 1. Thermal Resistance vs Air Flow Selectable 5V and 3.3V Figure 2 shows a regulator configuration which is pin selectable for 3.3V or 5V output. An external N-channel power MOSFET bypasses the regulator to provide 5V output. When the gate of the MOSFET is grounded by a pin on the microprocessor, the MOSFET turns off and the regulator supplies a 3.3V output. For this type of circuit to operate properly, the regulator must be designed to withstand 5V forced on its output pin without damage. All 20k 12V SHORT FOR 3.3V OPEN FOR 5V (SHORTING LINK IN µP PACKAGE) MTD3055EL (1.5A) MTB30N06EL (3A) MTB50N06EL (5A) 5V INPUT LINEAR REGULATOR 3.3V OR 5V OUTPUT AN58 • F02 Properly designed step-down, or buck, switching regulators can provide 5V to 3.3V conversion efficiencies as high as 95%. In a step-down switching regulator, the inductor current flows from the input when the switch is ON and through a diode (or synchronously switching FET) from ground when the switch is OFF. Keys to high efficiency include minimizing quiescent current, using a low resistance power MOSFET switch and in higher current applications, using a synchronous switch to reduce the diode losses. In continuous operation (i.e., the inductor current does not go to zero), the duty cycle for a 5V to 3.3V switching regulator is 66%. This means that the switch is ON for 2/3 of each cycle and OFF for the remaining 1/3. Table 2 shows four switching regulators suitable for 5V to 3.3V conversion. All of these regulators break the 90% efficiency barrier over a wide range of load currents. High efficiency makes for a compact layout and allows all surface mount solutions at high current since heat sinking is either modest or unnecessary. Figure 2. 3.3V Regulator with 5V Bypass Circuit AN58-3 Application Note 58 Power Supply Sequencing and Rise Time Table 2. 3.3V Switching Regulators LOAD CURRENT DEVICE SYNCHRO- SHUTDOWN NOUS CURRENT EFFICIENCY 200mA to 400mA LTC1174-3.3 No 1µA 90% 0.5mA to 2A LTC1147-3.3 Yes 10µA 92% 1A to 5A LTC1148-3.3 Yes 10µA 94% 5A to 20A LT1158 Yes 2.2mA 91% The LTC1174, LTC1147 and LTC1148 step-down, high efficiency switching regulators feature Burst ModeTM operation to maintain low quiescent current at light loads (sleep mode) and in the LTC1148, synchronous operation at higher output currents. The LTC1147 and LTC1148 use a constant off-time, current-mode architecture. This results in excellent line and load transient response, constant inductor ripple current and well controlled startup and short-circuit currents. New 3.3V microprocessors must interface with 5V logic circuits. As a precaution against damaging logic interfaces, supply turn-on characteristics must be controlled. For example, one specification calls for a maximum difference between the system (5V) supply and the microprocessor (3.3V) supplies of 2.25V. Not all of the circuits shown will meet this specification or have been characterized for input/output differential. The linear regulators in the LT1083 thru LT1086 family will maintain proper startup and shutdown voltages for mixed supply systems. On turn-on, the output follows the input less the 1.2V dropout voltage until 3.3V is reached on the output. At turn-off, an internal diode insures the 3.3V supply follows the 5V supply down. Recommended Circuit Design Example The LTC1174 and LTC1147 are nonsynchronous converters for applications under 1A. The LTC1148 is fully synchronous for improved efficiency in the 2A to 5A output range. In Figure 17 an LTC1147 is used for 1A output current. This circuit consumes less board space than the LTC1148 circuit of Figure 18, at the cost of 2.5% worse efficiency. The LT1158 is a half-bridge driver designed for 5V to 20A applications. At these current levels multiple paralleled MOSFETs are necessary to maintain high efficiency. The LT1158 is used in Figures 21 and 22. Figure 4 shows a recommended circuit for general purpose 5V to 3.3V conversion in desktop machines. Since the microprocessor draws only a fraction of the total system power, the 66% efficiency of a linear regulator gives acceptable performance. The system requirements are: The compactness of a switching regulator solution is appealing, but not all of the required components shrink as easily as semiconductors and resistors. In particular, coils and high value capacitors present special miniaturization problems. Coil size is limited by practical considerations of core volume, temperature rise, and window area. Very high power densities have been achieved through the use of ferrite cores and materials such as molypermalloy. Unfortunately, the selection of surface mount bobbins for “E” style split cores has lagged behind. This area is the focus of development work but product introductions are slow in coming. A list of surface mount component suppliers can be found in Appendix A of Application Note 54. Transients Loads: 200mA to 3A in 100ns and 3A to 100mA in 100ns Burst ModeTM is a trademark of Linear Technology Corporation AN58-4 VIN = 5V ±0.25V VOUT = 3.3V ±0.3V IOUT = 3A Maximum Circuit Height: 1.5" Bypass Option: 5V out through low resistance switch if 3.3V processor is not installed. The LT1085 fullfills these requirements with the output bypassing capacitors shown in Figure 4. The 5V bypass switch, detailed in Figures 2 and 3 can be added as an option. A pin on the 3.3V microprocessor serves as the ground shorting switch to disable the bypass circuit. Application Note 58 Thermal design, as previously discussed, would require a 6°C/W heat sink. The Thermalloy 7025B-MT or Aavid 533402 meet this requirement as well as the 1.5" maximum height requirement. response should be checked in the finished circuit to verify the layout and capacitor placement. Figure 6 shows the output voltage tracking as the system is powered up and down. Note that the 5V supply never out runs the 3.3V supply by more than approximately 1.2V, thereby protecting the processor from damage. Regulation, including all combinations of line, load, and temperature, is better than 1.6% (53mV) for the LT1085— well inside the 300mV specification. Figure 5 shows the transient response to a 3A load change. The transient Ground current for the LT1085 is just 5mA, even at 3A output current. LT1085-3.3 (3A) 5V INPUT + 3.3V OUTPUT + 10µF 100µF ALUMINUM ELECTROLYTIC + 100µF ALUMINUM ELECTROLYTIC + 10µF SOLID TANTALUM 10nF to 100nF (20 PLACES) AN58 • F04 Figure 4. Multiple Bypassing is Necessary in Order to Assure Good Transient Response VOUT 100mV/DIV IOUT 2A/DIV AN58 • F05 50µs/DIV Figure 5. Regulator Transient Response 1V/DIV 5V INPUT 1V/DIV 3.3V OUTPUT 5V INPUT 3.3V OUTPUT 2µs/DIV AN58 • F06a 6A. Power Up 10µs/DIV AN58 • F06b 6B. Power Down Figure 6. 5V/3.3V Tracking at Power Up and Power Down AN58-5 Application Note 58 CIRCUIT INDEX Linear Regulators Switching Regulators CURRENT DEVICE 125mA 150mA 700mA 800mA 1.5A 3A 5A 7.5A 10A LT1020/LT1120 LT1121 LT1129 LT1117 LT1086 LT1085 LT1084 LT1083 LT1087 5V INPUT 5 IN OUT + 10µF SHDN 3 CURRENT DEVICE FIGURE NUMBER PAGE NUMBER 7 8 9 10 11 11 12 12 13 AN58-6 AN58-6 AN58-6 AN58-6 AN58-7 AN58-7 AN58-7 AN58-7 AN58-7 175mA 425mA 500mA 1A 1A 2A 5A 7.5A (10Apk) 15A (20Apk) LTC1174 LTC1174 LTC1147 LTC1147 LTC1148 LTC1148 LTC1148 LTC1158 LTC1158 14 15 16 17 18 19 20 21 22 AN58-7 AN58-8 AN58-8 AN58-9 AN58-10 AN58-11 AN58-12 AN58-13 AN58-14 1nF FEEDBACK 1 = OFF 0 = ON PAGE NUMBER 4 LT1120 LT1020 ALUMINUM FIGURE NUMBER 200k 2 + 22µF 3.3V 125mA 8 5V INPUT OUT 1 3.3V 150mA + 100nF ALUMINUM 825k GND IN 1 LT1121-3.3 SHDN 5 GND PIN 5 < 0.25V > 2.8V NC OUTPUT OFF ON ON 1µF SOLID TANTALUM 3 AN58 • F07 Figure 7. The LT1120 and LT1020 Include On-Chip Comparator Functions. See Their Data Sheets for Details 5 5V INPUT 100nF IN OUT 1 + LT1129-3.3 SENSE SHDN 4 GND 3 PIN 4 < 0.25V > 2.8V NC OUTPUT OFF ON ON 3.3V 700mA 2 IN + 10µF OUT LT1117-3.3 ALUMINUM + 3.3V 800mA 22µF ALUMINUM GND AN58 • F10 AN58 • F09 Figure 9. The Output of the LT1129 Can Be Pulled Up to 5V with No Ill Effects AN58-6 Figure 8. The Output of the LT1121 Can Be Pulled Up to 5V with No Ill Effects 5V INPUT 3.3µF SOLID TANTALUM AN58 • F08 Figure 10. The LT1117 is Available in a Low Cost, SOT-223 Package Application Note 58 5V INPUT IN 3.3V, 1.5A (LT1086-3.3) 3.3V, 3A (LT1085-3.3) OUT LT1085-3.3 LT1086-3.3 + 10µF ALUMINUM 5V INPUT IN + 10µF ALUMINUM + 22µF SOLID TANTALUM GND 3.3V, 5A (LT1084) 3.3V, 7.5A (LT1083) OUT LT1083 LT1084 + 22µF SOLID TANTALUM ADJ 124Ω AN58 • F11 205Ω AN58 • F06 Figure 11. See Figure 4 in the Design Example for Practical Bypassing Values Figure 12. Five Components Deliver Up to 7.5A MASTER 5V INPUT + 1k SENSE – IN SENSE + 10µF ALUMINUM 100nF LT1087 RM 3.3V 10A OUT ADJ 61.9Ω SLAVE 1k + 22µF SOLID TANTALUM 102Ω SENSE + IN SENSE – 100nF LT1087 RS OUT ADJ AN58 • F13 RM = 16 SQUARES OF 1 0Z COPPER (8mΩ) RS = 14.6 SQUARES OF 1 0Z COPPER (7.3mΩ) Figure 13. Independently, LT1087s Handle 5A. Their Reference Sense Pins Force Current Sharing for Parallel Operation at 10A + 6 3 2 7 100nF VIN LBIN SHDN LBOUT VOUT IPGM SW LTC1174-3.3 100 3 × 15µF* 25V 90 8 1 5 50µH† + GND MBRS140T3 3.3V 175mA 2 × 33µF** 16V EFFICIENCY (%) 5V INPUT 80 70 4 L = 50µH VOUT = 3.3V IPGM = 0V COIL = CTX50-1 60 *AVX TPSD156K025 **AVX TPSD336K016 † COILTRONICS CTX50-1 AN58 • F14a 50 1 10 100 LOAD CURRENT (mA) 14A. 14B. 300 AN58 • F14b Figure 14. The LTC1174-3.3 Uses Burst ModeTM Operation Throughout its Entire Current Range. Shutdown Current is Just 1µA AN58-7 Application Note 58 100 7 3 2 + 6 VIN IPGM SHDN LBIN VOUT LBOUT SW 3× 15µF* 25V 8 0.1µF 90 EFFICIENCY (%) 5V INPUT 1 3.3V 425mA 5 50µ† LTC1174-3.3 + 2× 33µF** 16V MBRS140T3 GND 4 80 70 L = 50µH VOUT = 3.3V IPGM = VIN COIL = CTX50-4 60 AN58 • F15a *AVX TPSD156K025 **AVX TPSD336K016 † COILTRONICS CTX50-4 50 10 100 LOAD CURRENT (mA) 1 500 AN58 • F15b 15B. 15A. Figure 15. Pulling IPGM (Pin 7) High Increases the Internal Current Threshold and Output Current Capability 5V INPUT D1 MBRS140T3 Q1 Si9405DY C1 0.1µF 1 + C2 15µF 25V × 2 4 L1 100µH LTC1147-3.3 1 2 3 C4 270pF NPO C3 3300pF X7R 4 VIN PDRIVE CT GROUND ITH SHDN SENSE + SENSE – 2 8 7 6 SHUTDOWN 5 + C5 0.001µF R1 1k 3 R2 0.2Ω C6 220µF 10V AN58 • F16a C2: AVX (Ta) TPSD156K025R0200, ESR = 0.200Ω, IRMS = 0.775A C6: AVX (Ta) TPSE227K010R0080, ESR = 0.80Ω, IRMS = 1.285A Q1: SILICONIX BVDSS = 20V, DCRON = 0.14Ω, Qg = 29nC D1: MOTOROLA R2: KRL SP-1/2-A1-0R200, PD = 0.75W L1: COILTRONICS CTX100-4, DCR = 0.175Ω, Kool Mµ® Core VOUT 3.3V 0.5A 16A. EFFICIENCY (%) 100 90 80 70 1 10 100 1k LOAD CURRENT (mA) AN58 • F16b 16B. Figure 16. Slightly Improved Efficiency Over the Circuit of Figure 15, with Continuous Mode Operation at Higher Loads AN58-8 Application Note 58 5V INPUT + C1 0.1µF 6 1 VIN SHDN 8 P-DRIVE Q1 Si9430DY R1 1k 2 C3 3300pF X7R ITH CT C4 560pF NPO 1 4 LTC1147-3.3 3 C2 22µF × 2 25V SENSE + 5 SENSE – 4 L1 100µH 2 3 R2 0.1Ω C5 0.001µF 3.3V 1A + C6 220µF 10V D1 MBRS330 GND 7 AVX (Ta) TPSD226K025R0200 ESR = 0.200Ω IRMS = 0.775A AVX (Ta) TPSE227K010R0080 ESR = 0.080Ω IRMS = 1.285A SILICONIX BVDSS = 20V DCRON = 0.100Ω CRSS = 400pF Qg = 50nC MOTOROLA KRL SP-1/2-A1-0R100 Pd = 0.75W COILTRONICS CTX100-4 DCR = 0.175Ω KOOL Mµ® CORE QUIESCENT CURRENT = 170µA TRANSITION CURRENT (BURST MODETM OPERATION/CONTINUOUS OPERATION) = 170mA AN58 F17a Figure 17A. LTC1147 (5V to 3.3V/1A) Buck Converter with Surface Mount Technology. Shutdown Current is Just 10µA 100 90 EFFICIENCY (%) C2: C6: Q1: D1: R2: L1: 80 70 60 50 1 10 100 OUTPUT CURRENT (mA) 1000 AN58 F17b Figure 17B. LTC1147 (5V to 3.3V/1A) Buck Converter Measured Efficiency AN58-9 Application Note 58 5V INPUT + + C1 1µF C2 0.1µF 10 C3 22µF x 2 25V 3 VIN SHDN P-DRIVE 1 Q1 Si9430DY 1 4 6 R1 1k 4 C4 3300pF X7R C1: C3: C7: Q1: Q2: D1: R2: L1: ITH SENSE + LTC1148-3.3 SENSE – CT C5 560pF NPO N-DRIVE S-GND P-GND 11 12 (Ta) AVX (Ta) TPSD226K025R0200 ESR = 0.200Ω IRMS = 0.775A AVX (Ta) TPSE227K010R0080 ESR = 0.080Ω IRMS = 1.285A SILICONIX NMOS BVDSS = 20V RDSON = 0.100Ω CRSS = 400pF Qg = 50nC SILICONIX NMOS BV DSS = 30V RDSON = 0.050Ω CRSS = 160pF Qg = 30nC MOTOROLA SCHOTTKY VBR = 40V KRL SP-1/2-A1-0R100J Pd = 0.75W COILTRONICS CTX100-4 DCR = 0.175Ω KOOL Mµ® CORE L1 100µH 3 R2 100mΩ 3.3V 1A 8 7 C6 0.01µF 14 + Q2 Si9410DY D1 MBRS140T3 C7 220µF 10V QUIESCENT CURRENT = 180µA TRANSITION CURRENT (BURST MODETM OPERATION/CONTINUOUS OPERATION) = 250mA AN58 F18a ALL OTHER CAPACITORS ARE CERAMIC Figure 18A. LTC1148 (5V to 3.3V/1A) Fully Synchronous Buck Converter 100 EFFICIENCY (%) 90 80 70 60 50 1 10 100 OUTPUT CURRENT (mA) 1000 AN58 F18b Figure 18B. LTC1148 (5V to 3.3V/1A) Buck Converter Measured Efficiency AN58-10 2 Application Note 58 5V INPUT + + C2 0.1µF C1 1µF 10 6 4 R1 1k C4 3300pF X7R C5 470pF NPO 3 VIN SHUTDOWN ITH P-DRIVE SENSE + LTC1148-3.3 SENSE – CT S-GND N-DRIVE P-GND 11 C1: C3: C7: Q1: Q2: D1: R2: L1: 1 C3 22µF x 3 25V Q1 Si9430DY L1 50µH R2 50mΩ +3.3V 2A 8 7 C6 0.01µF + 14 Q2 Si9410DY D1 MBRS140T3 C7 220µF x 2 10V 12 (Ta) AVX (Ta) TPSD226K025R0200 ESR = 0.200Ω IRMS = 0.775A AVX (Ta) TPSE227K010R0080 ESR = 0.080Ω IRMS = 1.285A SILICONIX PMOS BVDSS = 20V RDSON = 0.100Ω CRSS = 400pF Qg = 50nC SILICONIX NMOS BVDSS = 30V RDSON = 0.050Ω CRSS = 160pF Qg = 30nC MOTOROLA SCHOTTKY VBR = 40V KRL SL-1-C1-0R050J Pd = 1W COILTRONICS CTX50-2-MP DCR = 0.032Ω MPP CORE (THROUGH HOLE) QUIESCENT CURRENT = 180µA TRANSITION CURRENT (BURST MODETM OPERATION/CONTINUOUS OPERATION) = 450mA AN58 F19a ALL OTHER CAPACITORS ARE CERAMIC Figure 19A. LTC1148 (5V to 3.3V/2A) Buck Converter 100 EFFICIENCY (%) 90 80 70 60 50 1 10 100 OUTPUT CURRENT (mA) 1000 2000 AN58 F19b Figure 19B. LTC1148 (5V to 3.3V/2A) Buck Converter Measured Efficiency AN58-11 Application Note 58 5V INPUT Q2 Si9430 Q1 Si9430 D1 MBRS140T3 + C3 100µF 20V × 2 Q3 Si9410 1 C2 0.1µF + C1 1µF 2 3 4 C5 680pF NPO 5 6 C4 3300pF X7R R1 470Ω 7 P-DRIVE N-DRIVE NC NC LTC1148-3.3 VIN P-GND CT S-GND INT VCC ITH SHDN NC SENSE – SENSE + 14 13 L1 27µH 12 11 10 SHUTDOWN 9 + 8 C6 0.01µF C7 220µF 6.3V × 2 R2 0.02Ω 3.3V 5A C1: Ta AN58 • F20a C3: SANYO (OS-CON) 20SA100M, ESR = 0.037Ω, IRMS = 2.25A C7: SANYO (OS-CON) 10SA220M, ESR = 0.035Ω, IRMS = 2.36A Q1, Q2: SILICONIX PMOS BVDSS = 20V, DCRON = 0.100Ω, Qg = 50nC Q3: SILICONIX NMOS BVDSS = 30V, DCRON = 0.050Ω, Qg = 30nC D1: MOTOROLA SCHOTTKY VBR = 30V R2: KRL NP-2A-C1-0R020J, PD = 3W L1: 17 TURNS #16 ON MAGNETICS 77120-A7 Figure 20A. LTC1148 (5V to 3.3V/5A) Buck Converter. Beyond 5A the LT1158 is a Better Choice EFFICIENCY (%) 100 90 80 70 1 10 100 1000 LOAD CURRENT (mA) 10000 AN58 F20b Figure 20B. LTC1148 (5V to 3.3V/5A) Buck Converter Measured Efficiency AN58-12 Application Note 58 5V INPUT BAT85* BOOST f = 50kHz + 10µF 0.01µF V+ T-DR V+ T-FB 10Ω 10Ω 510k 0.15µF 3.3k BIAS 0.01µF + SOURCE 3 8 24k 2 4 CMOS 555 LT1158 IN 6 B-DR 500pF 1 CIN 220µF 10V × 3 OS-CON B-FB 7 SENSE+ FAULT SENSE – 1000pF V+ 0.01µF COLL 16k 1.62k – LT1431 REF F-GND S-GND 220pF + 4.99k COUT 330µF 6.3V × 2 AVX 3.3V 7.5A OUTPUT RS 10mΩ + L1 22µH MOSFETS: MTB50N06EL (SURFACE MOUNT) IRLZ44 (THROUGH HOLE) L1: HURRICANE LAB HL-KK122T/BB RS: DALE WSC-2-0.01 (SURFACE MOUNT) LVR-3-0.01 (THROUGH HOLE) *PHILIPS BAT85 (THROUGH HOLE) CENTRAL CMPSH-3 (SURFACE MOUNT) AN58 F21a Figure 21A. High Efficiency High Current 5V to 3.3V Switching Regulator Output Current = 7.5A Cont. 10A Peak 100 90 EFFICIENCY (%) 0.33µF 80 70 60 50 0 1 2 3 4 5 6 7 8 OUTPUT CURRENT (A) 9 10 AN58 F21b Figure 21B. High Efficiency High Current 5V to 3.3V Switching Regulator AN58-13 Application Note 58 5V INPUT BAT85* BOOST f = 50kHz V+ + 10µF 0.01µF V 10Ω T-DR + 10Ω 10Ω 510k T-FB 0.22µF 3.3k BIAS 0.01µF + SOURCE 3 8 24k 2 4 CMOS 555 LT1158 IN 6 B-DR 500pF 1 10Ω CIN 220µF 10V × 4 OS-CON 10Ω B-FB 7 SENSE+ FAULT SENSE – 1000pF V+ COLL 0.01µF 16k 1.62k 0.33µF LT1431 – REF F-GND S-GND 220pF + 4.99k COUT 330µF 6.3V × 4 AVX 3.3V 15A OUTPUT RS 5mΩ + L1 9µH MOSFETS: MTB50N06EL (SURFACE MOUNT) IRLZ44 (THROUGH HOLE) L1: COILTRONICS CTX02-12171-1 RS: DALE WSC-2-0.005 (SURFACE MOUNT) LVR-3-0.005 (THROUGH HOLE) *PHILIPS BAT85 (THROUGH HOLE) CENTRAL CMPSH-3 (SURFACE MOUNT) Figure 22A. High Efficiency High Current 5V to 3.3V Switching Regulator Output Current = 15A Cont. 20A Peak 100 EFFICIENCY (%) 90 80 70 60 50 0 2 4 6 8 10 12 14 16 18 20 OUTPUT CURRENT (A) AN58 F22b Figure 22B. High Efficiency High Current 5V to 3.3V Switching Regulator AN58-14 AN58 F16a Application Note 58 APPENDIX The following photographs illustrate the effect of various types and values of output capacitance on the transient response of the LT1085. The current step for these photographs is equal to the worst case supply current change specified by Intel. This current step occurs when the processor transitions from an idle state to a running state or from a running state to an idle state. The current step illustrated in the photographs is from 100mA to 3A and then from 3A back to 100mA. Both transitions occur in 100ns. A number of different capacitor types and combinations were used. In each case the capacitors were chosen to limit the output voltage deviation to less than ±5% of 3.3V. For all photographs: 1) The top trace is the output variation and the vertical scale is equal to 100mV per division. 2) The bottom trace is the output current step at 2A per division. The horizontal scale for all photographs is 50µs per division. COUT = 100µF/10V AVX Tantalum, Surface Mount in Parallel with 100µF/16V Aluminum Electrolytic COUT = 100µF/10V AVX Tantalum, Surface Mount APXA2 APXA1 COUT = 100µF/10V AVX Tantalum, Surface Mount in Parallel with 390µF/16V Aluminum Electrolytic COUT = 2-100µF/10V AVX Tantalum, Surface Mount in Parallel APXA3 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. APXA4 AN58-15 Application Note 58 COUT = 100µF/16V OS-CON in Parallel with 220µF/16V Aluminum Electrolytic COUT = 220µF/10V OS-CON APXA5 APXA6 COUT = 22µF/20V OS-CON in Parallel with 390µF/16V Aluminum Electrolytic COUT = 100µF/16V OS-CON APXA7 APXA8 COUT = 100µF/16V OS-CON in Parallel with 100µF/16V Aluminum Electrolytic APXA9 Linear Technology Corporation AN58-16 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LT/GP 0993 10K REV 0 • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 1993