MIC2601 Datasheet, PDF (9/13 Pages) Micrel Semiconductor – 1.2A, 1.2MHz/2MHz Wide Input Range Integrated Switch Boost Regulator

Micrel, Inc.
Application Information
DC-to-DC PWM Boost Conversion
The MIC2601/2 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 through pulse-width
modulation (PWM).
10µH
V_IN
V_OUT
MIC2601/2
VIN
SW
6.65K
2.2µF
GND
0.1µF
EN
FB
VDD
SS
AGND PGND
0.1µF
10µF
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 MIC2601/2 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 MIC2601/2 OVP ckt will shut the
switch off saving itself and other sensitive circuitry
downstream.
MIC2601/2
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 MIC2601 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 MIC2601/2. Y5V values may be used,
but to offset their tolerance over temperature, more
capacitance is required.
Diode Selection
The MIC2601/2 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.
Input capacitor
A minimum 2.2μF ceramic capacitor is recommended for
designing with the MIC2601/2. Increasing input
capacitance will improve performance and greater noise
September 2009
9
M9999-090909-A