MIC2182 datasheet(19/28 Pages) MICREL

19 / 28 page

June 2000

MIC2182

Micrel

The input voltage ripple will primarily depend on the input

capacitors ESR. The peak input current is equal to the peak

inductor current, so:

∆VI

inductor(peak)

ESR(C )

=×

The input capacitor must be rated for the input current ripple.

The RMS value of input capacitor current is determined at the

maximum output current. Assuming the peak to peak induc-

tor ripple current is low:

(rms)

D (1 D)

OUT(max)

≈×

× −

The power dissipated in the input capacitor is:

(rms)

DISS(C )

ESR(C )

=×

Voltage Setting Components

The MIC2182-3.3 and MIC2182-5.0 ICs contain internal

voltage dividers that set the output voltage. The MIC2182

adjustable version requires two resistors to set the output

voltage as shown in Figure 13.

Error

Amp

MIC2182 [adj.]

VREF

1.245V

Figure 13. Voltage-Divider Configuration

The output voltage is determined by the equation:

REF

=× +









Where: V

REF for the MIC2182 is typically 1.245V.

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 voltage

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

efficiency of the power supply, especially at low output 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:

(R1 R2) I

divider

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 power

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

capacitors 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 bal-

ance 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 oscilla-

tions. 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.

Decoupling Capacitor Selection

The 4.7

µF decoupling capacitor is used to minimize noise on

the VDD pin. The placement of this capacitor is critical to the

proper operation of the IC. It must be placed right next to the

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