circuits, or can give incorrect scope readings because of in-
duced voltages in the scope probe.
Application Hints
The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse En-
gineering, and ferrite bobbin core for Renco.
INPUT CAPACITOR (CIN)
To maintain stability, the regulator input pin must be bypassed
with at least a 47 μF electrolytic capacitor. The capacitor's
leads must be kept short, and located near the regulator.
An inductor should not be operated beyond its maximum rat-
ed current because it may saturate. When an inductor begins
to saturate, the inductance decreases rapidly and the inductor
begins to look mainly resistive (the DC resistance of the wind-
ing). This will cause the switch current to rise very rapidly.
Different inductor types have different saturation characteris-
tics, and this should be kept in mind when selecting an in-
ductor.
If the operating temperature range includes temperatures be-
low −25°C, the input capacitor value may need to be larger.
With most electrolytic capacitors, the capacitance value de-
creases and the ESR increases with lower temperatures and
age. Paralleling a ceramic or solid tantalum capacitor will in-
crease the regulator stability at cold temperatures. For maxi-
mum capacitor operating lifetime, the capacitor's RMS ripple
current rating should be greater than
The inductor manufacturer's data sheets include current and
energy limits to avoid inductor saturation.
INDUCTOR RIPPLE CURRENT
When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a saw-
tooth type of waveform (depending on the input voltage). For
a given input voltage and output voltage, the peak-to-peak
amplitude of this inductor current waveform remains constant.
As the load current rises or falls, the entire sawtooth current
waveform also rises or falls. The average DC value of this
waveform is equal to the DC load current (in the buck regu-
lator configuration).
INDUCTOR SELECTION
All switching regulators have two basic modes of operation:
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulator per-
formance and requirements.
If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value is)
will be forced to run discontinuous if the load current is light
enough.
The LM2575 (or any of the Simple Switcher family) can be
used for both continuous and discontinuous modes of oper-
ation.
OUTPUT CAPACITOR
An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2575 using short pc board traces. Standard alu-
minum electrolytics are usually adequate, but low ESR types
are recommended for low output ripple voltage and good sta-
bility. The ESR of a capacitor depends on many factors, some
which are: the value, the voltage rating, physical size and the
type of construction. In general, low value or low voltage (less
than 12V) electrolytic capacitors usually have higher ESR
numbers.
The inductor value selection guides in Figure 3 through Figure
7 were designed for buck regulator designs of the continuous
inductor current type. When using inductor values shown in
the inductor selection guide, the peak-to-peak inductor ripple
current will be approximately 20% to 30% of the maximum DC
current. With relatively heavy load currents, the circuit oper-
ates in the continuous mode (inductor current always flowing),
but under light load conditions, the circuit will be forced to the
discontinuous mode (inductor current falls to zero for a period
of time). This discontinuous mode of operation is perfectly
acceptable. For light loads (less than approximately 200 mA)
it may be desirable to operate the regulator in the discontin-
uous mode, primarily because of the lower inductor values
required for the discontinuous mode.
The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output capac-
itor and the amplitude of the inductor ripple current (ΔIIND).
See the section on inductor ripple current in Application Hints.
The lower capacitor values (220 μF–680 μF) will allow typi-
cally 50 mV to 150 mV of output ripple voltage, while larger-
value capacitors will reduce the ripple to approximately 20 mV
to 50 mV.
The selection guide chooses inductor values suitable for con-
tinuous mode operation, but if the inductor value chosen is
prohibitively high, the designer should investigate the possi-
bility of discontinuous operation. The computer design soft-
ware Switchers Made Simple will provide all component
values for discontinuous (as well as continuous) mode of op-
eration.
Output Ripple Voltage = (ΔIIND) (ESR of COUT
)
To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
“high-frequency,” “low-inductance,” or “low-ESR.” These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.05Ω can cause instability in the regulator.
Tantalum capacitors can have a very low ESR, and should be
carefully evaluated if it is the only output capacitor. Because
of their good low temperature characteristics, a tantalum can
Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least ex-
pensive, the bobbin core type, consists of wire wrapped on a
ferrite rod core. This type of construction makes for an inex-
pensive inductor, but since the magnetic flux is not completely
contained within the core, it generates more electromagnetic
interference (EMI). This EMI can cause problems in sensitive
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