Definition of Terms

BUCK REGULATOR

A switching regulator topology in which a higher voltage is

converted to a lower voltage. Also known as a step-down

switching regulator.

BUCK-BOOST REGULATOR

A 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.

CATCH DIODE OR CURRENT STEERING DIODE

The diode which provides a return path for the load current

when the LM2575EP 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 in

capacitor heating, which directly affects the capacitor’s op-

erating lifetime. When used as a switching regulator output

filter, higher ESR values result in higher output ripple volt-

ages.

Most standard aluminum electrolytic capacitors in the

100 µF–1000 µF range have 0.5

Ω 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 0.15

Ω.

EQUIVALENT SERIES INDUCTANCE (ESL)

The pure inductance component of a capacitor (see Figure

16). 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 VOLTAGE

The AC component of the switching regulator’s output volt-

age. It is usually dominated by the output capacitor’s ESR

multiplied by the inductor’s ripple current (

∆I

to-peak value of this sawtooth ripple current can be deter-

mined by reading the Inductor Ripple Current section of the

Application hints.

CAPACITOR RIPPLE CURRENT

RMS value of the maximum allowable alternating current at

which a capacitor can be operated continuously at a speci-

fied temperature.

STANDBY QUIESCENT CURRENT (I

Supply current required by the LM2575EP when in the

standby mode (ON /OFF pin is driven to TTL-high voltage,

thus turning the output switch OFF).

INDUCTOR RIPPLE CURRENT (

∆I

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).

20113220

Note: Pin numbers are for the TO-220 package.

FIGURE 15. 1.2V to 55V Adjustable 1A Power Supply with Low Output Ripple

20113221

FIGURE 16. Simple Model of a Real Capacitor

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