National Semiconductor LM1575 Series, Руководство пользователя

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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 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.
Most standard aluminum electrolytic capacitors in the
100 µF–1000 µF range have 0.5
Ω
to 0.1
Ω
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
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 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
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).
01147520
Note: Pin numbers are for the TO-220 package.
FIGURE 15. 1.2V to 55V Adjustable 1A Power Supply with Low Output Ripple
01147521
FIGURE 16. Simple Model of a Real Capacitor
LM1575/LM2575/LM2575HV
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