MIC2199BML datasheet(12/14 Pages) MICREL

12 / 14 page

January 2010

M9999-011310

Micrel, Inc.

MIC2199

Voltage Setting Components

The MIC2199 requires two resistors to set the output voltage

as shown in Figure 6.

Error

Amp

MIC2199

VREF

0.8V

Figure 6. Voltage-Divider Configuration

The output voltage is determined by the equation:

1 R1

REF

=× +













Where: VREF for the MIC2199 is typically 0.8V.

A typical value of R1 can be between 3k and 10k. If R1 is

too large it may allow noise to be introduced into the volt-

age feedback loop. If R1 is too small in value it will decrease

theefficiencyofthepowersupply,especiallyatlowoutput

loads.

Once R1 is selected, R2 can be calculated using:

VR1

REF

−

Voltage Divider Power Dissipation

The reference voltage and R2 set the current through the

voltage divider.

DIVIDER

REF

The power dissipated by the divider resistors is:

P(R1 R2) I

DIVIDER

Efficiency Calculation and Considerations

Efficiencyistheratioofoutputpowertoinputpower.The

difference is dissipated as heat in the buck converter. Under

lightoutputload,thesignificantcontributorsare:

• SupplycurrenttotheMIC2199

• MOSFETgate-chargepower(includedintheIC

supply current)

• Corelossesintheoutputinductor

Tomaximizeefficiencyatlightloads:

• Usealowgate-chargeMOSFETorusethesmall-

estMOSFET,whichisstilladequateformaximum

output current.

• Useaferritematerialfortheinductorcore,which

has less core loss than an MPP or iron power

core.

Underheavyoutputloadsthesignificantcontributorstopower

loss are (in approximate order of magnitude):

• Resistiveon-timelossesintheMOSFETs

• SwitchingtransitionlossesintheMOSFETs

• Inductorresistivelosses

• Current-senseresistorlosses

• Inputcapacitorresistivelosses(duetothecapaci-

torsESR)

To minimize power loss under heavy loads:

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

Multiplying the gate charge by the on-resistance

givesafigureofmerit,providingagoodbalance

betweenlowandhighloadefficiency.

• Slowtransitiontimesandoscillationsonthevoltage

andcurrentwaveformsdissipatemorepowerduring

turn-onandturnoffoftheMOSFETs.Acleanlayout

will minimize parasitic inductance and capacitance

in the gate drive and high current paths. This will

allow the fastest transition times and waveforms

withoutoscillations.Lowgate-chargeMOSFETs

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 re-

sistance. However, using too small of a value will

requiremoreoutputcapacitorstofiltertheoutput

ripple, which will force a smaller bandwidth, 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

limitandwillrequirelargerMOSFETsandinductor

components.

• Use low-ESR input capacitors to minimize the

powerdissipatedinthecapacitorsESR.

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