MIC2870 Datasheet, PDF (19/25 Pages) Micrel Semiconductor – 1.5A Synchronous Boost Flash LED Driver with I2C Interface

Micrel, Inc.
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. Since the boost
converter is compensated internally, the recommended
inductance of L is limited from 1µH to 2.2µH to ensure
system stability. It is usually a good balance between
these considerations.
A large inductance value reduces the peak-to-peak
inductor ripple current hence the output ripple voltage
and the LED ripple current. This also reduces both the
DC loss and the transition loss at the same inductor’s DC
resistance (DCR). However, the DCR of an inductor
usually increases with the inductance in the same
package size. This is due to the longer windings required
for an increase in inductance. Since the majority of the
input current passes through the inductor, the higher the
DCR the lower the efficiency is, and more significantly at
higher load currents. On the other hand, inductor with
smaller DCR but the same inductance usually has a
larger size. The saturation current rating of the selected
inductor must be higher than the maximum peak inductor
current to be encountered and should be at least 20% to
30% higher than the average inductor current at
maximum output current.
Input Capacitor
A ceramic capacitor of 4.7µF or larger with low ESR is
recommended to reduce the input voltage ripple to
ensure a clean supply voltage for the device. The input
capacitor should be placed as close as possible to the
MIC2870 VIN pin with short trace for good noise
performance. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature. The
Y5V and Z5U type temperature rating ceramic capacitors
are not recommended due to their large reduction in
capacitance over temperature and increased resistance
at high frequencies. These reduce their ability to filter out
high-frequency noise. The rated voltage of the input
capacitor should be at least 20% higher than the
maximum operating input voltage over the operating
temperature range.
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing output capacitor
will lead to an improved transient response, however, the
size and cost also increase. The output capacitor is
preferred in the range of 2.2µF to 10µF with ESR from
10mΩ to 50mΩ. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature.
MIC2870
The Y5V and Z5U type ceramic capacitors are not
recommended due to their wide variation in capacitance
over temperature and increased resistance at high
frequencies. The rated voltage of the output capacitor
should be at least 20% higher than the maximum
operating output voltage over the operating temperature
range.
FRSET/TRSET Resistor
Since FRSET/TRSET resistor is used for setting the
maximum LED current in flash mode and torch mode
respectively, resistor type with 0.1% tolerance is
recommended for more accurate LED current setting.
September 5, 2013
19
Revision 1.0