MIC2182 datasheet(26/40 Pages) MICROCHIP

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MIC2182

DS20006644A-page 26



2022 Microchip Technology Inc. and its subsidiaries

The output voltage is determined by the equation:

EQUATION 5-33:

A typical value of R1 can be between 3 kΩ and 10 kΩ.

If R1 is too large, it may allow noise to be introduced

into the voltage feedback loop. If R1 is too small in

value, it will decrease the efficiency of the buck

converter, especially at low output loads.

Once R1 is selected, R2 can be calculated using:

EQUATION 5-34:

5.7.1

VOLTAGE DIVIDER POWER

DISSIPATION

The reference voltage and R2 set the current through

the voltage divider.

EQUATION 5-35:

The power dissipated by the divider resistors is:

EQUATION 5-36:

5.8

Efficiency Calculation and

Considerations

Efficiency is the ratio of output power to input power.

The difference is dissipated as heat in the buck

converter. Under light output load, the significant

contributors are:

• Supply current to the MIC2182

• MOSFET gate-charge power (included in the IC

supply current)

• Core losses in the output inductor

To maximize efficiency at light loads:

• Use a low gate-charge MOSFET or use the

smallest MOSFET, which is still adequate for

maximum output current.

• Allow the MIC2182 to run in skip mode at lower

currents.

• Use a ferrite material for the inductor core, which

has less core loss than an MPP or iron powder

core.

Under heavy output loads, the significant contributors

to power loss are (in approximate order of magnitude):

• Resistive on-time losses in the MOSFETs

• Switching transition losses in the MOSFETs

• Inductor resistive losses

• Current-sense resistor losses

• Input capacitor resistive losses (due to the

capacitor’s ESR)

To minimize power loss under heavy loads:

• Use logic-level, low on-resistance MOSFETs.

Multiplying the gate charge by the on-resistance

gives a figure of merit, providing a good balance

between low and high load efficiency.

• Slow transition times and oscillations on the

voltage and current waveforms dissipate more

power during turn-on and turnoff of the MOSFETs.

A clean layout will minimize parasitic inductance

and capacitance in the gate drive and high current

paths. This will allow the fastest transition times

and waveforms without oscillations. Low

gate-charge MOSFETs will transition faster than

those with higher gate-charge requirements.

• For the same size inductor, a lower value will

have fewer turns and therefore, lower winding

resistance. However, using too small of a value

will require more output capacitors to filter the

output ripple, which will force a smaller band-

width, slower transient response and possible

instability under certain conditions.

• Lowering the current-sense resistor value will

decrease the power dissipated in the resistor.

However, it will also increase the overcurrent limit

and will require larger MOSFETs and inductor

components.

• Use low-ESR input capacitors to minimize the

power dissipated in the capacitors ESR.

VOUT VREF 1 R1

-------











Where:

VREF for the MIC2182 is typically 1.245V

VREF R1



VOUT VREF

–

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

IDIVIDER

VREF

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

PDIVIDER

R1 R2





IDIVIDER



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