Micrel, Inc.

MIC2290

October 2007

8

M9999-101907

Application Information

DC-to-DC PWM Boost Conversion

The MIC2290 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

(D1). Voltage regulation is achieved through pulse-width

modulation (PWM).

L1

10µH

C2

10µF

R2

R1

MIC2290

VIN

VIN

VOUT

EN

SW

FB

GND

GND

OUT

GND

C1

2.2µF

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

1

D

−

=

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 MLF

® package option, there is an overvoltage

protection function. If the feedback resistors are discon-

nected from the circuit or the feedback pin is shorted to

ground, the feedback pin will fall to ground potential.

This will cause the MIC2290 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 MIC2290 OVP pin will shut the switch off

when an overvoltage 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.

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 MIC2290 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:

2π

I

L

V

V

F

OUT

OUT

2

IN

rhpz

×

×

×

=

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 MIC2290. 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

22µF

<16V

10µF

<34V

4.7µF

Table 1. Output Capacitor Selection