National Semiconductor LM1575 Series Manual
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Additional Applications (Continued)Definition of TermsBUCK REGULATORA switching regulator topology in which a higher voltage isconverted to a lower voltage. Also known as a step-downswitching regulator.BUCK-BOOST REGULATORA switching regulator topology in which a positive voltage isconverted to a negative voltage without a transformer.DUTY CYCLE (D)Ratio of the output switch’s on-time to the oscillator period.CATCH DIODE OR CURRENT STEERING DIODEThe diode which provides a return path for the load currentwhen the LM2575 switch is OFF.EFFICIENCY (η)The proportion of input power actually delivered to the load.CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)The purely resistive component of a real capacitor’s imped-ance (see Figure 16). It causes power loss resulting incapacitor heating, which directly affects the capacitor’s op-erating lifetime. When used as a switching regulator outputfilter, higher ESR values result in higher output ripple volt-ages.Most standard aluminum electrolytic capacitors in the100 μF–1000 μF range have 0.5Ω to 0.1Ω ESR. Higher-grade capacitors (“low-ESR”, “high-frequency”, or “low-inductance”’) in the 100 μF–1000 μF range generally haveESR of less than 0.15Ω.EQUIVALENT SERIES INDUCTANCE (ESL)The pure inductance component of a capacitor (see Figure16). The amount of inductance is determined to a largeextent on the capacitor’s construction. In a buck regulator,this unwanted inductance causes voltage spikes to appearon the output.OUTPUT RIPPLE VOLTAGEThe AC component of the switching regulator’s output volt-age. It is usually dominated by the output capacitor’s ESRmultiplied by the inductor’s ripple current (∆I IND). The peak-to-peak value of this sawtooth ripple current can be deter-mined by reading the Inductor Ripple Current section of theApplication hints.CAPACITOR RIPPLE CURRENTRMS value of the maximum allowable alternating current atwhich a capacitor can be operated continuously at a speci-fied temperature.STANDBY QUIESCENT CURRENT (I STBY )Supply current required by the LM2575 when in the standbymode (ON /OFF pin is driven to TTL-high voltage, thusturning the output switch OFF).INDUCTOR RIPPLE CURRENT (∆IIND)The peak-to-peak value of the inductor current waveform,typically a sawtooth waveform when the regulator is operat-ing in the continuous mode (vs. discontinuous mode).01147520Note: Pin numbers are for the TO-220 package.FIGURE 15. 1.2V to 55V Adjustable 1A Power Supply with Low Output Ripple01147521FIGURE 16. Simple Model of a Real CapacitorLM1575/LM2575/LM2575HVwww.national.com 22 |
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