Micrel

MIC2295

July 2004

M9999-052402

(408) 955-1690

9

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

10uH

D1

1A/40V

Schottky

C2

10uF

Vin

Gnd

Vout

MIC2288BML

R1

R2

Gnd

VIN

SW

FB

GND

EN

Figure 2. Typical Application

OVP

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 10uH 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

OUT

L I

OUT

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