MIC2185 datasheet(14/15 Pages) MICREL

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MIC2185

Micrel, Inc.

MIC2185

October 2005

• Use a ferrite material for the inductor core, which

has less core loss than an MPP or iron power

core.

The signiﬁcant contributors to power loss at higher output

loads are (in approximate order of magnitude):

• Resistive on-time losses in both MOSFETs

• Switching transition losses in the low side MOS-

FET

• Inductor resistive losses

• Current sense resistor losses

• Output 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 switching and resistive power dissipa-

tion.

• Slow transition times and oscillations on the

voltage and current waveforms dissipate more

power during the turn-on and turn-off of the

low side MOSFET. 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

switch faster than those with higher gate charge

speciﬁcations.

• 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 increase the inductor current and therefore

require more output capacitors to ﬁlter the output

ripple.

• Lowering the current sense resistor value will

decrease the power dissipated in the resistor.

However, it will also increase the overcurrent

limit and may require larger MOSFETs and

inductor components to handle the higher cur-

rents.

• Use low ESR output capacitors to minimize the

power dissipated in the capacitor’s ESR.

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