MIC2291

Micrel

M9999-081104

6

August, 2004

Applications 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 current

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 dis-

charged 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.

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;

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 MIC2291, there is an over voltage

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 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 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

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;

frhpz

V

VL I

2

IN

2

OUT

OUT

=

××

× π

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 in-

creasing the output capacitor value (decreasing loop gain).

Output Capacitor

A 1

µF or greater output capacitor is sufficient for most

designs. An X5R or X7R dielectric ceramic capacitors are

recommended for designs with the MIC2291. Y5V values

may be used, but to offset their tolerance over temperature,

more capacitance is required.

Diode Selection

The MIC2291 requires an external diode for operation. A

schottkey 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, the maximum output current and the maximum

reverse voltage is rated greater than the output voltage.

Input Capacitor

A minimum 1

µF ceramic capacitor is recommended for

designing with the MIC2291. Increasing input capacitance

will improve performance and greater noise immunity on the

source. The input capacitor should be as close as possible to

D1

V

V

IN

OUT

=−

10

µH

C2

1

µF

1-Cell

Li Ion

R2

MIC2291-34BML

VIN

EN

SW

OVP

FB

GND

D1

1A/40V

Schottky

3xLED

GND

GND