MIC2874 Datasheet, PDF (17/22 Pages) Micrel Semiconductor – 1.2A High-Brightness Flash LED Driver with Single-Wire Serial Interface

Micrel, Inc.
MIC2874
Component Selection
Inductor
Inductor selection should strike 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 0.47µH to
1µH to ensure system stability, and a 0.47µH inductor is
typically recommended. 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 MIC2874 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, and a 4.7µF ceramic capacitor is typically
recommended. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature. 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.
Power Dissipation Consideration
As with all power devices, the ultimate current rating of the
output is limited by the thermal properties of the device
package and the PCB on which the device is mounted.
There is a simple, Ohm’s law type relationship between
thermal resistance, power dissipation and temperature
which are analogous to an electrical circuit:
Figure 5. Series Electrical Resistance Circuit
From this simple circuit we can calculate V_X if we know
I_SOURCE, V_Z and the resistor values, R_XY and R_YZ using
Equation 1:
V_X = I_SOURCE × (R _XY + R _YZ ) + V_Z
Eq. 1
Thermal circuits can be considered using this same rule
and can be drawn similarly by replacing current sources
with power dissipation (in watts), resistance with thermal
resistance (in °C/W) and voltage sources with temperature
(in °C).
Figure 6. Series Thermal Resistance Circuit
Now replacing the variables in the equation for V_X, we can
find the junction temperature (T_J) from the power
dissipation, ambient temperature and the known thermal
resistance of the PCB (θ_CA) and the package (θ_JC).
August 26, 2014
17
Revision 1.0