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

diodescannotbeignored.Poorchoicesforthesecomponents

can cause problems as severe as circuit failure or as subtle

as poorer than expected efficiency.

a.

b.

c.

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

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

alsobeveryslow,especiallythosesuitedtopowerrectification

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

L

× t

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

output switch is off, the voltage across the inductor is:

For normal operation the inductor current is a triangular

waveform which returns to zero current (discontinuous mode)