Micrel

MIC2295

April 2005

9

M9999-042605

(408) 955-1690

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.

L1

10mH

C2

10µF

R2

R1

MIC2288BML

VIN

EN

SW

FB

GND

GND

OVP

GND

C1

2.2µF

D1

Figure 2

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;

D

= 1−

V

IN

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.

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 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

MIC2295 OVP pin will shut the switch off when an over-

voltage 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. 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;

Frhpz

=

V

IN

2

V

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

MIC2295. 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

Recommended Output

Capacitance

<6V

10µF

<16V

4.7µF

<34V

2.2µF