August 2007 7 MIC2570
MIC2570 Micrel, Inc.
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 varia-
tions 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
conditions.
Inductors, Capacitors and Diodes
The importance of choosing correct inductors, capacitors and
diodes can not be ignored. Poor choices for these components
can cause problems as severe as circuit failure or as subtle
as poorer than expected efficiency.
a.
b.
c.
Inductor Current
Time
Figure 3. Inductor Current: a. Normal,
b. Saturating, and c. Excessive ESR
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 significantly
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.
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.
4.75
5.00
5.25
0 500 1000 1500
OUTPUT VOLTAGE (V)
TIME (µs)
Figure 4. Output Ripple
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 rectification
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:
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:
When the output switch turns off, the voltage across the in-
ductor changes sign and flies high in an attempt to maintain
a constant current. The inductor voltage will eventually be
clamped to a diode drop above V
OUT
. Therefore, when the
output switch is off, the voltage across the inductor is:
V
2
= V
OUT
+ V
DIODE
– V
IN
For normal operation the inductor current is a triangular
waveform which returns to zero current (discontinuous mode)
