MIC2606 datasheet(11/20 Pages) MICROCHIP

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

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DS20006620A-page 11

MIC2605/6

4.0

APPLICATION INFORMATION

4.1

DC-to-DC PWM Boost Conversion

The MIC2605 and MIC2606 are constant frequency

boost converters. They operate by taking a DC input

voltage and regulating a higher DC output voltage.

Figure 4-1 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. Voltage

regulation is achieved through pulse-width modulation

(PWM).

FIGURE 4-1:

Typical Application Circuit.

4.2

Duty Cycle Considerations

Duty cycle refers to the switch on-to-off time ratio and

can be calculated as follows for a boost regulator:

EQUATION 4-1:

The duty cycle required for voltage conversion should

be less than the maximum duty cycle of 85% for the

MIC2605 and 80% for the MIC2606. 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.

4.3

Overvoltage Protection

There is an overvoltage protection function for both the

MIC2605 and the MIC2606. If the output voltage

overshoots the set voltage by 15% when feedback is

high during input higher than output, turn on, load

transients, line transients, load disconnection, etc. The

MIC2605 or MIC2606 OVP circuit will shut the switch

off, protecting itself and other sensitive circuitry

downstream.

4.4

Component Selection

4.4.1

INDUCTOR

Inductor selection is a balance between efficiency,

stability, cost, size, and rated current. For most

applications, a 10 μH 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 MIC2605/6 operating currents) 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:

EQUATION 4-2:

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

either

reducing

inductance

(increasing RHPZ frequency) or increasing the output

capacitor value (decreasing loop gain).

VIN

VOUT

-------------

–

FRHPZ

1 D

–



2



2  L IO



----------------------------------

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