April 2005 7 M9999-042205
MIC2288 Micrel, Inc.
Applications Information
DC-to-DC PWM Boost Conversion
The MIC2288 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. 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 Schottky diode (D1). Voltage regulation is achieved
by modulating the pulse width or pulse-width modulation
(PWM).
L1
10µH
C2
10µF
R2
R1
MIC2288BML
VIN
V
IN
V
OUT
EN
SW
FB
GND
GND
OVP
GND
C1
2.2µF
D1
Figure 2. Typical Application Circuit
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can be
calculated as follows for a boost regulator:
D1
V
V
IN
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.
Overvoltage Protection
For the MLF™ package option, there is an overvoltage
protection function. If the feedback resistors are discon-
nected from the circuit or the feedback pin is shorted to
ground, the feedback pin will fall to ground potential. This will
cause the MIC2288 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 MIC2288 OVP pin will shut the
switch off when an overvoltage 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.
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 MIC2288 operating current) is passed
through the inductor, higher DCR inductors will reduce effi-
ciency.
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
V
VLI2
rhpz
IN
OUT 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 in-
creasing the output capacitor value (decreasing loop gain).
Output Capacitor
Output capacitor selection is also a trade-off between perfor-
mance, 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 MIC2288. 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 Recomended Output Capacitance
<6V 22µF
<16V 10µF
<34V 4.7µF
Table 1. Output Capacitor Selection
Diode Selection
The MIC2288 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.
