1997

7

MIC2571

MIC2571

Micrel

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.

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 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.

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 200m

Ω ESR can cause 100mV of ripple at the

output due the capacitor only. Acceptable values of ESR are

typically in the 50m

Ω 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

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

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:

V = V

– V

1IN

SAT