Datasheet

 2018 Microchip Technology Inc. DS20006034B-page 11
MIC2288
5.0 APPLICATION INFORMATION
5.1 DC/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 5-1 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).
FIGURE 5-1: Typical Application Circuit.
5.2 Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and
can be calculated as follows for a boost regulator:
EQUATION 5-1:
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.
5.3 Overvoltage Protection
For the DFN package option, there is an overvoltage
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 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 the regulator and other sensitive
circuitry downstream.
5.4 Component Selection
5.4.1 INDUCTOR
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. For most
applications, 10 H 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. Because the majority of input
current (minus the MIC2288 operating current) is
passed through the inductor, higher DCR inductors will
reduce efficiency.
To maintain stability, increasing the inductor value will
have to be associated 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:
EQUATION 5-2:
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).
L1
10μH
C2
10μF
R2
R1
MIC2288YML
VIN
V
IN
V
OUT
EN
SW
FB
GND
GND
OVP
GND
C1
2.2μF
D1
D 1
V
IN
V
OUT
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–=
f
RHPZ
V
IN
2
V
OUT
L I
OUT
 2
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