MIC2253_11 Datasheet, PDF (9/13 Pages) Micrel Semiconductor – 3.5A 1MHz High Efficiency Boost Regulator with OVP and Softstart

Micrel, Inc.
Component Selection
Inductor
The MIC2253 is designed to work with a 2.2µH inductor.
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:
frhpz =
V_IN ²
V_OUT × 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 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 MIC2253.
The output capacitor sets the frequency of the dominant
pole and zero in the power stage. The zero is given by:
1
fz = _{C × Resr × 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 MIC2253 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:
fp =
I_OUT
C × V_OUT × 2 × π
Diode Selection
The MIC2253 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
MIC2253
inductor current and the maximum reverse voltage is
rated greater than the output voltage.
Input Capacitor
A minimum 2.2µF ceramic capacitor with an X5R or X7R
dielectric is recommended for designing with the
MIC2253. 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 MIC2253, with short traces for good
noise performance.
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:
fzero = ¹
2πR ₂C ₄
The resistor should be set to approximately 600Ω. The
capacitor typically ranges from 10nF to 100nF.
Adding an optional capacitor from comp pin-to-ground
adds a pole at approximately:
fpole = ¹
2πR ₂C₃
This capacitor typically is 100pF. 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 MIC2253 evaluation board document for
component location.
Feedback Resistors
The feedback pin (FB) provides the control path to the
control the output. The FB pin is used to compare the
output to an internal reference. Output voltages are
adjusted by selecting the appropriate feedback network
values. The desired output voltage can be calculated as
follows:
V_OUT
=
V_REF
⋅
⎜⎜⎝⎛
R₁
R₂
+ 1_{⎟⎟⎠⎞}
where V_REF is equal to 1.245V.
April 2011
9
M9999-042011-C