MIC2290_07 Datasheet, PDF (8/13 Pages) Micrel Semiconductor – 2mm × 2mm PWM Boost Regulator with Internal Schotty Diode

Micrel, Inc.
Application Information
DC-to-DC PWM Boost Conversion
The MIC2290 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 internal Schottky diode
(D1). Voltage regulation is achieved through pulse-width
modulation (PWM).
L1
V_IN
10µH
V_OUT
C1
2.2µF
GND
MIC2290
VIN SW
OUT
EN
FB
GND
R1
C2
10µF
R2
GND
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:
D = 1 − ^VIN
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.
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 MIC2290 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 MIC2290 OVP pin will shut the switch off
October 2007
MIC2290
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.
Large inductance values reduce the peak-to-peak ripple
current, affecting efficiency. This has an 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 MIC2290 operating current) is passed through the
inductor, higher DCR inductors will reduce efficiency.
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_rhpz
=
V_OUT
V_IN ²
× L × I_OUT
× 2π
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 MIC2290. 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
<6V
<16V
<34V
Recommended Output Capacitance
22µF
10µF
4.7µF
Table 1. Output Capacitor Selection
8
M9999-101907