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MIC4574 Datasheet(PDF) 5 Page - Micrel Semiconductor |
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MIC4574 Datasheet(HTML) 5 Page - Micrel Semiconductor |
5 / 6 page MIC4574 Micrel 4-96 April 1998• Functional Description The MIC4574 is a variable duty cycle switch-mode regulator with an internal power switch. Refer to the block diagrams. Supply Voltage The MIC4574 operates from a +4V to +24V unregulated input. Highest efficiency operation is from a supply voltage below +15V. Enable/Shutdown The shutdown (SHDN) input is TTL compatible. Ground the input if unused. A logic-low enables the regulator. A logic- high shuts down the internal regulator which reduces the current to typically 50 µA. Feedback Fixed versions of the regulator have an internal resistive divider from the feedback (FB) pin. Connect FB directly to the output line. Adjustable versions require an external resistive voltage divider from the output voltage to ground, connected from the 1.23V tap to FB. Duty Cycle Control A fixed-gain error amplifier compares the feedback signal with a 1.23V bandgap voltage reference. The resulting error amplifier output voltage is compared to a 200kHz 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 com- parator output. A lower feedback voltage increases the duty cycle. Output Switching When the internal switch is on, an increasing current flows from the supply V IN, through external storage inductor L1, 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 L1 forces current to flow through fast recovery diode D1, charging C OUT. Output Capacitor External output capacitor C OUT provides stabilization and reduces ripple. 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 L1, which means that D1 is part of the high-current return path. Applications Information The applications circuits that follow have been constructed and tested. Refer to Application Note 15 for additional information, including efficiency graphs and manufacturer’s addresses and telephone numbers for most circuits. For a mathematical approach to component selection and circuit design, refer to Application Note 14. C1 47µF 35V D1 1N5819 C2 220µF 16V L1 100µH SGND FB SW MIC4574-3.3BN SHDN 6V to 24V 3.3V/0.5A VIN C1 Nichicon UPL1V470MEH, ESR = 0.34 Ω C2 Nichicon UPL1C221MPH, ESR = 0.16 Ω D1 Motorola 1N5819 L1 Sumida RCH875-101K, DCR = 0.28 Ω PGND 24 1 7 3 5 Figure 1. 6V–24V to 3.3V/0.5A Buck Converter Through Hole C1 47µF 35V D1 1N5819 C2 220µF 16V L1 100µH SGND FB SW MIC4574-5.0BN SHDN 8V to 24V 5.0V/0.5A VIN C1 Nichicon UPL1J470MEH, ESR = 0.34 Ω C2 Nichicon UPL1C221MPH, ESR = 0.16 Ω D1 Motorola 1N5819 L1 Sumida RCH875-101K, DCR = 0.28 Ω PGND 24 1 7 3 5 Figure 2. 8V–24V to 5V/0.5A Buck Converter Through Hole C1 33µF 63V MBR160 C2 220µF 16V L1 220µH SGND FB SW MIC4574BN SHDN 16V to 24V 12V/0.5A VIN R2 13.0k 1% R1 1.50k 1% C1 Nichicon UPL1J330MEH, ESR = 0.35 Ω C2 Nichicon UPL1C221MPH, ESR = 0.16 Ω D1 Motorola MBR160 L1 Sumida RCH106-221K, DCR = 0.78 Ω PGND 1 7 4 2 5 3 Figure 3. 16V–24V to 12V/0.5A Buck Converter Through Hole |
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