MIC2182 datasheet(23/40 Pages) MICROCHIP

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DS20006644A-page 23

MIC2182

5.3.1

RMS CURRENT AND MOSFET

POWER DISSIPATION

CALCULATION

Under normal operation, the high-side MOSFET’s RMS

current is greatest when VIN is low (maximum duty

cycle). The low-side MOSFET’s RMS current is

greatest when VIN is high (minimum duty cycle).

However, the maximum stress to the MOSFETs occurs

during short circuit conditions, where the output current

equal

IOVERCURRENT(MAX). (See the

Current-Sense Resistor Selection section). The

calculations below are for normal operation. To

calculate the stress under short circuit conditions,

substitute IOVERCURRENT(MAX) for IOUT(MAX). Use the

formula below to calculate duty cycle D under short

circuit conditions.

EQUATION 5-16:

The RMS value of the high-side switch current is:

EQUATION 5-17:

The RMS value of the low-side switch current is:

EQUATION 5-18:

EQUATION 5-19:

Converter

efficiency

depends

component

parameters, which have not yet been selected. For

design purposes, an efficiency of 90% can be used for

VIN less than 10V and 85% can be used for VIN greater

than 10V. The efficiency can be more accurately

calculated once the design is complete. If the assumed

efficiency is grossly inaccurate, a second iteration

through the design procedure can be made.

For the high-side switch, the conduction power loss is:

EQUATION 5-20:

Because the AC switching losses for the low-side

MOSFET is near zero, the total power dissipation of the

low-side MOSFET is:

EQUATION 5-21:

The total power dissipation for the high-side MOSFET

is:

EQUATION 5-22:

DSHORTCIRCUIT 0.063 1.8 10

–

VIN



–

ISWHS RMS





IOUT MAX





IL PP







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















ISWLS RMS





1 D

–





IOUT MAX





IL PP







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















Where:

D = Duty Cycle of the Converter

VOUT



VIN



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

Where:

= Efficiency of the Converter

PCOND SWHS





RDSON HS





ISWHS RMS







Where:

RDSON(HS) = High-side MOSFET ON-Resistance

PD SWLS





PCOND SWLS





RDSON LS





ISWLS RMS







Where:

RDSON(LS) = Low-side MOSFET ON-Resistance

PD SWHS





PCOND SWHS





PAC SWHS





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