Micrel MIC2295
April 2005
9
M9999-042605
(408) 955-1690
Application Information
DC to DC PWM Boost Conversion
The MIC2295 is a constant frequency boost converter. It
operates by taking a DC input voltage and regulating a
higher DC output voltage. Figure 2 shows a typical circuit.
L1
10mH
C2
10µF
R2
R1
MIC2288BML
VIN
V
IN
V
OUT
EN
SW
FB
GND
GND
OVP
GND
C1
2.2µF
D1
Figure 2
Boost regulation is achieved by turning on an internal
switch, which draws current through the inductor (L1).
When the switch turns off, the inductor’s magnetic field
collapses, causing the current to be discharged into the
output capacitor through an external Schottkey diode (D1).
Voltage regulation is achieved my modulating the pulse
width or pulse width modulation (PWM).
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can
be calculated as follows for a boost regulator;
D = 1−
V
IN
V
OUT
The duty cycle required for voltage conversion should be
less than the maximum duty cycle of 85%. Also, in light
load conditions where the input voltage is close to the
output voltage, the minimum duty cycle can cause pulse
skipping. This is due to the energy stored in the inductor
causing the output to overshoot slightly over the regulated
output voltage. During the next cycle, the error amplifier
detects the output as being high and skips the following
pulse. This effect can be reduced by increasing the
minimum load or by increasing the inductor value.
Increasing the inductor value reduces peak current, which
in turn reduces energy transfer in each cycle.
Over Voltage Protection
For MLF package of MIC2295, there is an over voltage
protection function. If the feedback resistors are
disconnected from the circuit or the feedback pin is
shorted to ground, the feedback pin will fall to ground
potential. This will cause the MIC2295 to switch at full
duty-cycle in an attempt to maintain the feedback voltage.
As a result the output voltage will climb out of control. This
may cause the switch node voltage to exceed its maximum
voltage rating, possibly damaging the IC and the external
components. To ensure the highest level of protection, the
MIC2295 OVP pin will shut the switch off when an over-
voltage condition is detected saving itself and other
sensitive circuitry downstream.
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size and rated current. For most applications
a 10µH is the recommended inductor value. It is usually a
good balance between these considerations. Efficiency is
affected by inductance value in that larger inductance
values reduce the peak to peak ripple current. This has an
effect of reducing both the DC losses and the transition
losses.
There is also a secondary effect of an inductors 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
MIC2295 operating current) is passed through the
inductor, higher DCR inductors will reduce efficiency.
Also, 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;
Frhpz =
V
IN
2
V
OUT
×L × I
OUT
× 2
π
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
MIC2295. Y5V values may be used, but to offset their
tolerance over temperature, more capacitance is required.
The following table shows the recommended ceramic
(X5R) output capacitor value vs. output voltage.
Output Voltage Recommended Output
Capacitance
<6V 10µF
<16V 4.7µF
<34V 2.2µF
