MIC2605_09 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 0.5A, 1.2MHz / 2MHz Wide Input Range Boost Regulator with Integrated Switch and Schottky Diode

Micrel, Inc.
Application Information
DC-to-DC PWM Boost Conversion
The MIC2605/6 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.
Voltage regulation is achieved through pulse-width
modulation (PWM).
10µH
V_OUT
32V, 30mA
V_IN = 12V
1µF
0.1µF
MIC2605/6
VIN
SW
OUT
EN
FB
VDD
SS PGND
0.1µF
12.4K
1µF
499
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 MIC2605/6 there is an over voltage protection
function. If the output voltage overshoots the set voltage
by 15% when feedback is high during input higher than
output, turn on, load transients, line transients, load
disconnection etc. the MIC2605/6 OVP ckt will shut the
switch off saving itself and other sensitive circuitry
downstream.
MIC2605/6
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 MIC2605/6 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:
FRHPZ = ^{(D)2 ⋅VO}
2 ⋅ π ⋅ L ⋅ I_O
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 MIC2605/6. Y5V values may be used,
but to offset their tolerance over temperature, more
capacitance is required.
Input capacitor
A minimum 1μF ceramic capacitor is recommended for
designing with the MIC2605/6. Increasing input
capacitance will improve performance and greater noise
immunity on the source. The input capacitor should be
as close as possible to the inductor and the MIC2605/6,
with short traces for good noise performance.
September 2009
10
M9999-090909-B