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Figure 2 shows an example of inductor current in the continu-
ous mode with its associated change in oscillator frequency
and duty cycle. This situation is most likely to occur with
relatively small inductor values, large input voltage variations
and output voltages which are less than ~3× the input voltage.
Selection of an inductor with a saturation threshold above
1.2A will insure that the system can withstand these condi-
tions.
Inductors, Capacitors and Diodes
The importance of choosing correct inductors, capacitors and
diodes can not be ignored. Poor choices for these compo-
nents can cause problems as severe as circuit failure or as
subtle as poorer than expected efficiency.
Micrel
Capacitors
It is important to select high-quality, low ESR, filter capacitors
for the output of the regulator circuit. High ESR in the output
capacitor causes excessive ripple due to the voltage drop
across the ESR. A triangular current pulse with a peak of
500mA into a 200mΩ ESR can cause 100mV of ripple at the
output due the capacitor only. Acceptable values of ESR are
typically in the 50mΩ range. Inexpensive aluminum electro-
lytic capacitors usually are the worst choice while tantalum
capacitors are typically better. Figure 4 demonstrates the
effect of capacitor ESR on output ripple voltage.
5.25
OUTPUT VOLTAGE (V)
a.
Inductor Current
5.00
b.
c.
4.75
Time
0
500
1000
TIME (µs)
1500
Figure 3. Inductor Current: a. Normal,
b. Saturating and c. Excessive ESR
Figure 4. Output Ripple
Inductors
Inductors must be selected such that they do not saturate
under maximum current conditions. When an inductor satu-
rates, its effective inductance drops rapidly and the current
can suddenly jump to very high and destructive values.
Figure 3 compares inductors with currents that are correct
and unacceptable due to core saturation. The inductors have
the same nominal inductance but Figure 3b has a lower
saturation threshold. Another consideration in the selection
of inductors is the radiated energy. In general, toroids have
the best radiation characteristics while bobbins have the
worst. Some bobbins have caps or enclosures which signifi-
cantly reduce stray radiation.
The last electrical characteristic of the inductor that must be
considered is ESR (equivalent series resistance). Figure 3c
shows the current waveform when ESR is excessive. The
normal symptom of excessive ESR is reduced power transfer
efficiency. Note that inductor ESR can be used to the
designers advantage as reverse battery protection (current
limit) for the case of relatively low output power one-cell
designs. The potential for very large and destructive currents
exits if a battery in a one-cell application is inserted back-
wards into the circuit. In some applications it is possible to
limit the current to a nondestructive (but still battery draining)
level by choosing a relatively high inductor ESR value which
does not affect normal circuit performance.
Output Diode
Finally, the output diode must be selected to have adequate
reverse breakdown voltage and low forward voltage at the
application current. Schottky diodes typically meet these
requirements.
Standard silicon diodes have forward voltages which are too
large except in extremely low power applications. They can
also be very slow, especially those suited to power rectifica-
tion such as the 1N400x series, which affects efficiency.
Inductor Behavior
The inductor is an energy storage and transfer device. Its
behavior (neglecting series resistance) is described by the
following equation:
I =
V
×
t
L
where:
V = inductor voltage (V)
L = inductor value (H)
t = time (s)
I = inductor current (A)
If a voltage is applied across an inductor (initial current is
zero) for a known time, the current flowing through the
inductor is a linear ramp starting at zero, reaching a maximum
value at the end of the period. When the output switch is on,
the voltage across the inductor is:
V
1
= V
IN
– V
SAT
1997
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MIC2571
MIC [ MIC GROUP RECTIFIERS ]
