LM2574 datasheet(21/24 Pages) NSC

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Additional Applications (Continued)

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 LM2574 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 17). It causes power loss resulting in ca-

pacitor heating, which directly affects the capacitor’s operat-

ing lifetime. When used as a switching regulator output filter,

higher ESR values result in higher output ripple voltages.

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

17). The amount of inductance is determined to a large ex-

tent 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

IND). The peak-

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

STBY)

Supply current required by the LM2574 when in the standby

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

ing the output switch OFF).

INDUCTOR RIPPLE CURRENT (

∆I

IND)

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).

CONTINUOUS/DISCONTINUOUS MODE OPERATION

Relates 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.

DS011394-24

Note: Pin numbers are for 8-pin DIP package.

FIGURE 16. 1.2V to 55V Adjustable 500 mA Power Supply with Low Output Ripple

DS011394-25

FIGURE 17. Simple Model of a Real Capacitor

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