MIC2282 Datasheet, PDF (6/10 Pages) Micrel Semiconductor – Single-Cell Ultra Low EMI Boost LED Driver

Micrel, Inc.
Current "ratchet"
without current limit
Current limit threshold
Continuous current
Discontinuous current
Time
Figure 1. Current Limit Behavior
Figure 1 shows an example of inductor current in the
continuous 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 1.1A 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.
MIC2282
2b 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 2c 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 backwards 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 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 electrolytic
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
5.00
c.
Time
Figure 2. 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 saturates, its effective inductance decreases
rapidly and the current can suddenly jump to very high
values.
Figure 2 compares inductors with currents that are
correct and unacceptable due to core saturation. The
inductors have the same nominal inductance but Figure
4.75
0
500
1000
TIME (µs)
1500
Figure 3. 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.
October 2009
6
M9999-102309