EQUIVALENT SERIES INDUCTANCE (ESL)
Definition of Terms
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 un-
wanted inductance causes voltage spikes to appear on the
output.
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.
OUTPUT RIPPLE VOLTAGE
BUCK-BOOST REGULATOR
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 (ΔIIND). 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.
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.
CAPACITOR RIPPLE CURRENT
RMS 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 LM2575 when in the standby
mode (ON /OFF pin is driven to TTL-high voltage, thus turning
the output switch OFF).
CATCH DIODE OR CURRENT STEERING DIODE
The diode which provides a return path for the load current
when the LM2575 switch is 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).
EFFICIENCY (η)
The proportion of input power actually delivered to the load.
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.
CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)
The purely resistive component of
a real capacitor's
INDUCTOR SATURATION
impedance (see Figure 16). It causes power loss resulting in
capacitor heating, which directly affects the capacitor's oper-
ating lifetime. When used as a switching regulator output filter,
higher ESR values result in higher output ripple voltages.
The 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 domi-
nates. Inductor current is then limited only by the DC resis-
tance 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.
1147521
FIGURE 16. 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-induc-
tance”') in the 100 μF–1000 μF range generally have ESR of
less than 0.15Ω.
23
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