MIC2298 Datasheet, PDF (10/13 Pages) Micrel Semiconductor – 3.5A Minimum, 1MHz Boost High Brightness White LED Driver

Micrel, Inc.
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. For most
applications, a 4.7µH is the recommended inductor
value. It is usually a good balance between these
considerations. Larger inductance values reduce the
peak-to-peak ripple current, affecting efficiency. This has
the effect of reducing both the DC losses and the
transition losses. There is also a secondary effect of an
inductor’s DC resistance (DCR). The DCR of an inductor
will be higher for more inductance in the same package
size. This is due to the longer windings required for an
increase in inductance. Since the majority of input
current (minus the MIC2298 operating current) is passed
through the inductor, higher DCR inductors will reduce
efficiency. To maintain stability, increasing inductor size
will have to be met with an increase in output
capacitance. This is due to the unavoidable “right half
plane zero” effect for the continuous current boost
converter topology. The frequency at which the right half
plane zero occurs can be calculated as follows:
f _rhpz
=
V_OUT
V_IN ²
⋅ L ⋅ I_OUT
⋅ 2π
The right half plane zero has the undesirable effect of
increasing gain, while decreasing phase. This requires
that the loop gain is rolled off before this has significant
effect on the total loop response. This can be
accomplished by either reducing inductance (increasing
RHPZ frequency) or increasing the output capacitor
value (decreasing loop gain).
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing output
capacitance will lead to an improved transient response,
but also an increase in size and cost. X5R or X7R
dielectric ceramic capacitors are recommended for
designs with the MIC2298.
The output capacitor sets the frequency of the dominant
pole and zero in the power stage. The zero is given by:
f_z
=
1
C ⋅ R_esr
⋅ 2π
For ceramic capacitors, the ESR is very small. This puts
the zero at a very high frequency where it can be
ignored. Fortunately, the MIC2298 is current mode in
operation which reduces the need for this output
capacitor zero when compensating the feedback loop.
The frequency of the pole caused by the output
capacitor is given by.
f_p
=
I_OUT
C ⋅V_OUT ⋅ 2 ⋅ π
October 2007
MIC2298
Diode Selection
The MIC2298 requires an external diode for operation. A
Schottky diode is recommended for most applications
due to their lower forward voltage drop and reverse
recovery time. Ensure the diode selected can deliver the
peak inductor current and the maximum reverse voltage
is rated greater than the output voltage.
Input capacitor
A minimum 1µF ceramic capacitor with an X5R or X7R
dielectric is recommended for designing with the
MIC2298. Increasing input capacitance will improve
performance and greater noise immunity on the source.
The input capacitor should be as close as possible to the
inductor and the MIC2298, with short traces for good
noise performance.
The MIC2298 utilizes a feedback pin to compare the
LED current to an internal reference. The LED current is
adjusted by selecting the appropriate feedback resistor
value. The desired output current can be calculated as
follows:
I_LED
=
0.2V
R
Compensation
The comp pin is connected to the output of the voltage
error amplifier. The voltage error amplifier is a
transconductance amplifier. Adding a series RC-to-
ground adds a zero at:
f _zero
=
1
2πR ₂C ₄
The resistor typically ranges from 1kΩ to 50kΩ. The
capacitor typically ranges from 1nF to 100nF.
Adding an optional capacitor from comp pin-to-ground
adds a pole at approximately
f _pole
=
1
2πR ₂C ₃
This capacitor typically ranges from 100pF to 10nF.
Generally, an RC to ground is all that is needed. The RC
should be placed as close as possible to the
compensation pin. The capacitor should be a ceramic
with a X5R, X7R, or COG dielectric. Refer to the
MIC2298 evaluation board document for component
location.
10
M9999-101707-B