MIC4680 Evaluation Board

Transcription

1 SuperSwitcher SOP-8 Buck Switching Regulator 00kHz V to V/A General Description The MIC680 SuperSwitcher is a series of easy-to-use fixed and adjustable BiCMOS step-down (buck) switching regulators. The 00kHz MIC680 achieves A of continuous output current over the entire input voltage and temperature range, ( 0 C to +5 C) and up to 60 C ambient respectively, in a tiny SOP-8 package. It has a logic compatible enable that provides 5µA of quiescent current (typical) in shutdown mode. Efficiencies up to 8% peak are also possible. The MIC680 will also achieve up to.a of continuous output current over a V IN range of 0V to 0V and a 5V output. The MIC680 features a 00kHz switching frequency that reduces the inductor of the popular 5kHz, LM57x by a factor of, freeing up precious board space. The MIC680 is a thirdgeneration simple step-down switching regulator, with its lineage traced back to the popular LM57x, but has been enhanced with 00kHz operation, tighter current limit and thermal shutdown. Other improvements includes.8v of head room (V SAT ) and 0µA instead of 00µA (typical) quiescent current in shutdown mode. Requirements The MIC680 evaluation board requires a power supply capable of at least.7a at up to V. The load should be capable delivering.a under normal operation or A in current limit. Operation Figure shows the schematic of the evaluation board circuit. When the internal high-side switch turns on, one side of the inductor is fed from the input voltage, charging the inductor (+) and ( ). During this period, current flows from the input, through the internal switch, output inductor and load. When the output switch turns off, the inductor polarity switches to ( ) and (+), the SW pin voltage drops until the freewheeling diode is forward biased. During this portion of the cycle, current flows through the diode, inductor and load. Figure shows the 5V output efficiency versus input voltage and output current. Precautions MIC680 has no protection from reversed polarity being applied to its input. Any momentary reversal of the dc power supply connections can cause permanent damage to the circuit. Use extreme care with these connections. The safest way to power up the MIC680 evaluation board is to set the power supply to zero volts, and then gradually increase the supply voltage. Monitor the input supply current while increasing the input voltage. If the circuit draws excessive current with no load applied (greater than 00mA) then there is probably a problem with the set-up. Immediately shut off the main power supply and check for proper power supply connections. This simple procedure can avoid most catastrophic failures. Warning: Tantalum capacitors may explode if improperly connected. Always wear safety glasses when operating the evaluation board. EFFICIENCY (%) 5V Output Efficiency V 60 V V OUTPUT CURRENT (A) Figure. J V IN V to +V C 5µF 5V J OFF ON C 0.µF 50V S NKK GAP U MIC680BM IN SW SHDN SOP FB * C can be used to provide additional stability and improved transient response. D B60A or SS6 L 68µH R6 optional R.0k R 6.9k JPa.8V C* optional R.9k 5 JPb.5V 6 R.78k 7 JPc.V 8 R5 976Ω JPd 5.0V C 0µF 0V J V OUT A C5 0.µF 50V J Figure. Evaluation Board Schematic SuperSwitcher is a trademark of, Inc., Inc. 89 Fortune Drive San Jose, CA 95 USA tel + (08) fax + (08) August 000

2 Functional Description The MIC680 is a variable duty cycle switch-mode regulator with an internal power switch. Supply Voltage The MIC680 operates from a +V to +V unregulated input. Highest efficiency operation is from a supply voltage around +5V. See Figure. Enable/Shutdown The shutdown (SHDN) input is TTL compatible. Ground the input if unused. A logic-low enables the regulator. A logichigh shuts down the internal regulator which reduces the current to typically.5µa when V SHDN = V IN = V and 0µA when V SHDN = 5V. Feedback Fixed-voltage versions of the regulator have an internal resistive divider from the feedback (FB) pin. Connect FB directly to the output voltage. Adjustable versions require an external resistive voltage divider from the output voltage to ground, center tapped to the FB pin. See Table for recommended resistor values. Duty Cycle Control A fixed-gain error amplifier compares the feedback signal with a.v bandgap voltage reference. The resulting error amplifier output voltage is compared to a 00kHz sawtooth waveform to produce a voltage controlled variable duty cycle output. A higher feedback voltage increases the error amplifier output voltage. A higher error amplifier voltage (comparator inverting input) causes the comparator to detect only the peaks of the sawtooth, reducing the duty cycle of the comparator output. A lower feedback voltage increases the duty cycle. The MIC680 uses a voltage-mode control architecture. Output Switching When the internal switch is on, an increasing current flows from the supply V IN, through external storage inductor L, to output capacitor C OUT and the load. Energy is stored in the inductor as the current increases with time. When the internal switch is turned off, the collapse of the magnetic field in L forces current to flow through fast recovery diode D, charging C OUT. Output Capacitor External output capacitor C OUT provides stabilization and reduces ripple. See Bode Plots for additional information. Return Paths During the on-portion of the cycle, the output capacitor and load currents return to the supply ground. During the off portion of the cycle, current is being supplied to the output capacitor and load by storage inductor L, which means that D is part of the high-current return path. See Figure. MIC680BM IN SW SHDN FB N8.nF 8Ω D Figure. Increasing Maximum Output Current at High Input Voltages V IN +V to +V C IN IN MIC680BM SW SHDN FB Power SOP D L 68µH C OUT R R V OUT Load Figure. Critical Traces for Layout August 000

3 Bode Plots The following bode plots show that the MIC680 is stable over all conditions using a 68µF inductor (L) and a 0µF output capacitor (C OUT ). To assure stability, it is a good practice to maintain a phase margin of greater than 5. No-Load Stability Phase Margin = 06 Full-Load Stability Phase Margin = L = 68µF C OUT = 0µF V IN = 7V V OUT = 5.0V I OUT = 0.0A L = 68µF C OUT = 0µF V IN = 7V V OUT = 5.0V I OUT =.0A TIME (00ms/div.) TIME (00ms/div.) No-Load Stability Phase Margin = 7 Full-Load Stability Phase Margin = 69 L = 68µF C OUT = 0µF V IN = V V OUT = 5.0V I OUT = 0.0A L = 68µF C OUT = 0µF V IN = V V OUT = 5.0V I OUT =.0A TIME (00ms/div.) TIME (00ms/div.) Functional Characteristics V SW (NORMAL) V IN, 5V/A OUT V SW (SHORTED) V IN, 0V OUT Switching Frequency Foldback 00kHz 60kHz TIME Normal Operation Short Circuit Operation Frequency Foldback The MIC680 folds the switching frequency back during a hard shortcircuit condition to reduce the energy per cycle and protect the device. OUTPUT CURRENT (A) Safe Operating Area V OUT = 5V T A = 60 C Demonstration board layout Minimum Current Limit Note INPUT VOLTAGE (V) August 000

4 Applications Information Adjustable Regulators Adjustable regulators require a.v feedback signal. Recommended voltage-divider resistor values for common output voltages are included in Table. For other voltages, the resistor values can be determined using the following formulas: R () VOUT = VREF + R SHUTDOWN ENABLE V IN C IN MIC680BM IN SW SHDN FB 5 8 D L R R V OUT C OUT () R R V OUT = V REF Figure 5. Adjustable Regulator Circuit VREF =.V V O UT R * R*.8V.0k 6.9k.5V.0k.9k.V.0k.78k 5.0V.0k 976Ω 6.0V.0k 787Ω C IN D L C OUT 5µF 5V AVX TPSE5605R000 * All resistors % ** shielded magnetics for low RFI applications A 60V Schottky General Semiconductor SS6 68µH.5A Coiltronics UP-680 or Sumida CDRH5-680MC** or Sumida CDRH-680MC** Table. Recommended Components for Common Ouput Voltages 0µF 0V AVX TPSE700R0060 Thermal Considerations The MIC680 SuperSwitcher features the power-sop-8. This package has a standard 8-lead small-outline package profile but with much higher power dissipation than a standard SOP-8. The MIC680 SuperSwitcher is the first dc-to-dc converter to take full advantage of this package. The power SOP-8 has higher power dissipation (lower thermal resistance) because pins 5 though 8 and the die-attach SOP-8 θ JC θ JA θ CA AMBIENT ground plane heat sink area paddle are a single piece of metal. The die is attached to the paddle with thermally conductive adhesive. This provides a low thermal resistance path from the junction of the die to the ground pins. This design significantly improves package power dissipation by allowing excellent heat transfer through the ground leads to the printed circuit board. One of the limitation to the maximum output current on any MIC680 design is the junction-to-ambient thermal resistance (θ JA ) of the design (package and ground plane). Examining θ JA in more detail: θ JA = (θ JC + θ CA ) where: θ JC = junction-to-case thermal resistance θ CA = case-to-ambient thermal resistance θ JC is a relatively constant 0 C/W for a power SOP-8. θ CA is dependent on layout and is primarily governed by the connection of pins 5 though 8 to the ground plane. The purpose of the ground plane is to function as a heat sink. θ JA is ideally 6 C/W but will vary depending on the size of the ground plane to which the power SOP-8 is attached. printed circuit board Figure 6. Power SOP-8 Cross Section August 000

5 Determining Ground-Plane Heat-Sink Area There are two methods of determining the minimum ground plane area required by the MIC680. Quick Method Make sure that MIC680 pins 5 though 8 are connected to a ground plane with a minimum area of 6cm. This ground plane should be as close to the MIC680 as possible. The area maybe distributed in any shape around the package or on any pcb layer as long as there is good thermal contact to pins 5 though 8. This ground plane area is more than sufficient for most designs. Minimum Copper/Maximum Current Method Using Figure 7, for a given input voltage range, determine the minimum ground-plane heat-sink area required for the application s maximum output current. Figure 5 assumes a constant die temperature of 75 C above ambient. OUTPUT CURRENT (I) V V V V AREA (cm ) T A = 50 C Minimum Current Limit =.A Figure 7. Output Current vs. Ground Plane Area When designing with the MIC680, it is a good practice to connect pins 5 through 8 to the largest ground plane that is practical for the specific design. Checking the Maximum Junction Temperature: For this example, with an output power (P OUT ) of 5W, (5V output at A maximum with V IN = V) and 65 C maximum ambient temperature, determine the maximum junction temperature. Referring to the Figure, read the efficiency (η) for A output current at V IN = V or perform you own measurement. η = 79% The efficiency is used to determine how much of the output power (P OUT ) is dissipated in the regulator circuit (P D ). P D = P OUT ( η) P D = 5W ( 0.79) P D =.05W A worst-case rule of thumb is to assume that 80% of the total output power dissipation is in the MIC680 (P D(IC) ) and 0% is in the diode-inductor-capacitor circuit. P D(IC) = 0.8 P D P D(IC) = W P D(IC) = 0.8W Calculate the worst-case junction temperature: T J = P D(IC) θ JC + (T C T A ) + T A(max) where: T J = MIC680 junction temperature P D(IC) = MIC680 power dissipation θ JC = junction-to-case thermal resistance. The θ JC for the MIC680 s power-sop-8 is approximately 0 C/W. (Also see Figure.) T C = pin temperature measurement taken at the entry point of pins 6 or 7 into the plastic package at the ambient temperature (T A ) at which T C is measured. T A = ambient temperature at which T C is measured. T A(max) = maximum ambient operating temperature for the specific design. Calculating the maximum junction temperature given a maximum ambient temperature of 65 C: T J = C/W + (5 C 5 C) + 65 C T J = 0.8 C This value is less than the allowable maximum operating junction temperature of 5 C as listed in Operating Ratings. Typical thermal shutdown is 60 C. Increasing the Maximum Output Current The maximum output current at high input voltages can be increased for a given board layout. The additional three components shown in Figure will reduce the overall loss in the MIC680 by about 0% at high V IN and high I OUT. Even higher output current can be achieved by using the MIC680 to switch an external FET. See data sheet. Layout Considerations Layout is very important when designing any switching regulator. Rapidly changing switching currents through the printed circuit board traces and stray inductance can generate voltage transients which can cause problems. To minimize stray inductance and ground loops, keep trace lengths, indicated by the heavy lines in Figure 7, as short as possible. For example, keep D close to pin and pins 5 through 8, keep L away from sensitive node FB, and keep C IN close to pin and pins 5 though 8. The feedback pin trace from the output back to the IC should be kept as far way from the switching elements (usually L and D) as possible. A circuit with sample layouts are provided. See Figure 8b though 8e. August 000 5

6 Printed Circuit Board Layouts Figure 8a. Top-Side Silk Screen Figure 8c. Bottom-Side Silkscreen Figure 8b. Top-Side Copper Figure 8d. Bottom-Side Copper 6 August 000

7 Bill of Material Reference Part Number Manufacturer Description Qty C TPSD56M05R000 AVX 5µF 5V ECE-AHFS70 Panasonic 7µF 50V, 8mm.5mm C, C C0KATA AVX 0.µF 50V C AVX or other optional 00pF () C TPSD7M00R050 AVX 0µF 0V D SS6 General Semiconductor Schottky J J MillMax turret pins JP ND straight dual-row male header JP (Note ) ND female jumper header R.0k /0W %, size 0805 R 6.9k /0W %, size 0805 R.9k /0W %, size 0805 R.78k /0W %, size 0805 R5 976Ω /0W %, size 0805 R6 optional, size 0805 S GAP NKK Switches SPDT L UP-680 Coiltronics 68µH,.5A U MIC680BM Semiconductor A 00kHz power-so-8 buck regulator Note. Voltage selector. J V IN V to +V C 5µF 5V J OFF ON C 0.µF 50V S NKK GAP U MIC680BM IN SHDN SOP SW FB * C can be used to provide additional stability and improved transient response. D SS6 L 68µH R6 optional R.0k R 6.9k JPa.8V C* optional R.9k 5 JPb.5V 6 R.78k 7 JPc.V 8 R5 976Ω JPd 5.0V C 0µF 0V J V OUT A C5 0.µF 50V J Figure 8e. Evalution Board Schematic August 000 7

8 MICREL INC. 89 FORTUNE DRIVE SAN JOSE, CA 95 USA TEL + (08) FAX + (08) WEB This information is believed to be accurate and reliable, however no responsibility is assumed by for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Inc. 000 Incorporated 8 August 000