Micrel, Inc.

MIC2601/2

September 2009

9

M9999-090909-A

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

6.65K

MIC2601/2

VIN

EN

SW

FB

PGND

2.2µF

10µF

SS

0.1µF

VDD

AGND

0.1µF

GND

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:

OUT

IN

V

V

D

−

= 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.

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.

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:

( )

O

O

I

L

V

D

FRHPZ

⋅

⋅

⋅

⋅

=

π

2

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).

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