MIC2295 Micrel Semiconductor, MIC2295 Datasheet - Page 9

• Manufacturer Part Number: MIC2295
• Description: High Power Density 1.2A Boost Regulator
• Manufacturer: Micrel Semiconductor

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.
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;
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
July 2004
Vin
Micrel
Gnd
EN
Figure 2. Typical Application
10uH
L1
MIC2288BML
GND
VIN
D = 1
SW
FB
OVP
V
V
OUT
R1
IN
R2
Schottky
1A/40V
D1
10uF
C2
Vout
Gnd
9
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.
protection, the MIC2295 OVP pin will shut the switch
off when an over-voltage condition is detected
saving
downstream.
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size and rated current. For most
applications a 10uH 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;
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
itself
Frhpz =
To ensure the highest level of
and
V
OUT
other
L I
V
IN
2
OUT
sensitive
2
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M9999-052402
MIC2295
circuitry