MIC2150 Datasheet, PDF (15/27 Pages) Micrel Semiconductor – 2-Phase Dual Output PWM Synchronous Buck Control IC

Micrel, Inc.
Application Information
Passive Component Selection Guide
Inductor Selection
The inductor value is responsible for the ripple current
which causes some proportion of the resistive losses in
the power components. These losses are proportional to
I_RIPPLE2. Minimizing inductor ripple current can therefore
reduce the RMS current flowing in the power
components and generally improve efficiency; this is
achieved by choosing a larger value inductor. Having
said this, the actual value of inductance is realistically
defined by space limitations, RMS rating (I_RMS) and
saturation current (I_SAT) of available inductors. If one
looks at the newer flat wire inductors for example, these
typically have higher I_SAT ratings than the I_RMS for lower
values. Also, as inductance value increases, these
figures tend to get closer in value. This mirrors what
happens in the converter with I_SAT analogous to the
maximum peak switch current and I_RMS analogous to
output current. As inductance increases, so I_SWITCH(PK)
tends towards I_OUT. This is a characteristic that makes
these types of inductor optimal for use with high power
buck converters such as MIC2150.
To determine the I_SAT and I_RMS rating of the inductor, we
should start with a nominal value of ripple current. This
should typically be no more than I_OUT(MAX)/2 to minimize
MOSFET losses due to ripple current mentioned earlier.
Therefore:
L_MIN
~ 2 × ^VOUT
I_OUT × F_S
× (1−
V_IN
V_OUT
)
× Efficiency
I_LRMS > 1.04×I_OUT(MAX)
I_LSAT > 1.25×I_OUT(MAX)
Any value chosen above L_MIN will ensure these ratings
are not exceeded.
In considering the actual value to choose, one needs to
look at the effect of ripple on the other components in
the circuit. The chosen inductor value will have a ripple
current of:
I_RIPPLE ~
(1− D) _× V_OUT
F_S
L
This value should ideally be kept to a minimum, within
the cost and size constraints of the design, to reduce
unnecessary heat dissipation.
Output Capacitor Selection
The output capacitor (C_OUT) will have the full inductor
ripple current I_RIPPLERMS flowing through it. This creates
the output switching noise which consists of two main
components:
MIC2150
VOUT_{PK −PK}
≈ I_RIPPLE
× ESR +
I_RIPPLE × t_ON
2 × C_OUT
If therefore, the need is for low output voltage noise
(e.g., in low output voltage converters), V_OUT ripple can
be directly reduced by increasing inductor value, output
capacitor value or reducing ESR.
For tantalum capacitors, ESR is typically >40mΩ which
usually makes loop stabilization easier by utilizing a
pole-zero (type II) compensator.
Due to many advantages of multi-layer ceramic
capacitors, among them, cost, size, ripple rating and
ESR, it can be useful to choose these in many cases.
However, one disadvantage is the CV product. This is
lower than tantalum. A mixture of one tantalum and one
ceramic can be a good compromise which can still utilize
the simple type II compensator.
With ceramic output capacitors only, a double-pole,
double-zero (type III) compensator is required to ensure
system stability. Loop compensation is described in
more detail later in the data sheet.
Ensure the RMS ripple current rating of the capacitor is
above I_RIPPLE×0.6 to improve reliability.
Input Capacitor Selection
C_IN ripple rating for a single phase converter is typically
I_OUT/2 under worst case duty cycle conditions of 50%.
This increases ~10% for a ripple current of I_OUT/2.
When both cycles are switching 180° out of phase, the
ripple can reduce at DC <50% to:
I_RMSCIN = I₁₂ × D₁ × (1− D₁) + I₂₂ × D₂ × (1− D₂ ) − 2 × I₁ × I₂ × D₁ × D₂
It is however, also advisable to closely decouple the
Power MOSFETs with 2×10µF ceramic capacitors to
reduce ringing and prevent noise related issues from
causing problems in the layout of the regulator. The
ripple rating of C_IN may therefore, be satisfied by these
decoupling capacitors; allowing the use of perhaps one
more ceramic or tantalum input capacitor at the input
voltage node to decouple input noise and localize high
di/dt signals to the regulator input.
Power MOSFET Selection
The MIC2150 drives N-Channel MOSFETs in both the
upper and lower positions. This is because the switching
speed for a given R_DSON in the N-Channel device is
superior to the P-Channel device.
There are different criteria for choosing the upper and
lower MOSFETs and these criteria are more marked at
lower duty cycles such as 12V to 1.8V conversion. In
such an application, the upper MOSFET is required to
switch as quickly as possible to minimize transition
August 2009
15
M9999-082809-A
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