LM2576, LM2576HV
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SNVS107C –JUNE 1999–REVISED APRIL 2013
DEFINITION OF TERMS
BUCK REGULATORA switching regulator topology in which a higher voltage is converted to a lower voltage.
Also known as a step-down switching regulator.
BUCK-BOOST REGULATORA switching regulator topology in which a positive voltage is converted to a
negative voltage without a transformer.
DUTY CYCLE (D)Ratio of the output switch's on-time to the oscillator period.
(9)
CATCH DIODE OR CURRENT STEERING DIODEThe diode which provides a return path for the load current
when the LM2576 switch is OFF.
EFFICIENCY (η)The proportion of input power actually delivered to the load.
(10)
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)The purely resistive component of a real capacitor's
impedance (see Figure 34). It causes power loss resulting in capacitor heating, which directly affects the
capacitor's operating lifetime. When used as a switching regulator output filter, higher ESR values result in
higher output ripple voltages.
Figure 34. Simple Model of a Real Capacitor
Most standard aluminum electrolytic capacitors in the 100 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 have ESR of less than 0.15Ω.
EQUIVALENT SERIES INDUCTANCE (ESL)The pure inductance component of a capacitor (see Figure 34).
The amount of inductance is determined to a large extent on the capacitor's construction. In a buck
regulator, this unwanted inductance causes voltage spikes to appear on the output.
OUTPUT RIPPLE VOLTAGEThe AC component of the switching regulator's output voltage. It is usually
dominated by the output capacitor's ESR multiplied by the inductor's ripple current (ΔIIND). The peak-to-
peak value of this sawtooth ripple current can be determined by reading the INDUCTOR RIPPLE
CURRENT section.
CAPACITOR RIPPLE CURRENTRMS value of the maximum allowable alternating current at which a capacitor
can be operated continuously at a specified temperature.
STANDBY QUIESCENT CURRENT (ISTBY)Supply current required by the LM2576 when in the standby mode
(ON /OFF pin is driven to TTL-high voltage, thus turning the output switch OFF).
INDUCTOR RIPPLE CURRENT (ΔIIND)The peak-to-peak value of the inductor current waveform, typically a
sawtooth waveform when the regulator is operating in the continuous mode (vs. discontinuous mode).
CONTINUOUS/DISCONTINUOUS MODE OPERATIONRelates to the inductor current. In the continuous mode,
the inductor current is always flowing and never drops to zero, vs. the discontinuous mode, where the
inductor current drops to zero for a period of time in the normal switching cycle.
INDUCTOR SATURATIONThe condition which exists when an inductor cannot hold any more magnetic flux.
When an inductor saturates, the inductor appears less inductive and the resistive component dominates.
Inductor current is then limited only by the DC resistance of the wire and the available source current.
OPERATING VOLT MICROSECOND CONSTANT (E•Top)The product (in VoIt•μs) of the voltage applied to the
inductor and the time the voltage is applied. This E•Top constant is a measure of the energy handling
capability of an inductor and is dependent upon the type of core, the core area, the number of turns, and
the duty cycle.
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