Micrel, Inc.

MIC2291

May 2007

7

M9999-051507

Application Information

DC to DC PWM Boost Conversion

The MIC2291 is a constant frequency boost converter. It

operates by taking a DC input voltage and regulating

cur-rent through series LED’s by monitoring voltage

across the sense resistor (R2). LED current 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). Regulation is

then achieved by pulse width modulation (PWM) to

maintain a constant voltage on the FB pin. This in turn

provides constant LED current.

10µH

1µF

1-Cell

Li Ion

R2

MIC2291-34xML

VIN

EN

SW

OVP

FB

GND

3xLED

Schottky

1A/40V

D1

C2

GND

GND

Figure 2. DC to DC PWM Boost Conversion

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 volt-age, 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

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 MIC2291 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 MIC2291 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 MIC2291

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;

π

2

I

L

V

V

frhpz

OUT

OUT

2

IN

×

×

×

=

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).