Definition Of Terms - National Semiconductor LM1575 Series Manual

Additional Applications
Note: Pin numbers are for the TO-220 package.
FIGURE 15. 1.2V to 55V Adjustable 1A Power Supply with Low Output Ripple

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 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 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.
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(Continued)
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
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 LM2575 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).
22
FIGURE 16. Simple Model of a Real Capacitor
01147520
01147521
). The peak-
IND
)
STBY
)
IND