November 2004

13

MIC2198

MIC2198

Micrel

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:

II

D (1 D)

C (rms)

OUT(max)

IN

≈

××

−

The power dissipated in the input capacitor is:

PI

R

DISS(C )

C (rms)

ESR(C )

IN

IN

2

IN

=×

Voltage Setting Components

The MIC2198 requires two resistors to set the output voltage

as shown in Figure 6.

Error

Amp

3

MIC2198

FB

0.8V

R2

R1

Figure 6. Voltage-Divider Configuration

The output voltage is determined by the equation:

VV

1

R1

R2

O

REF

=×

+

⎛

⎝

⎜

⎞

⎠

⎟

Where: V

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:

R2

VR1

VV

REF

O

REF

=

×

−

Voltage Divider Power Dissipation

The reference voltage and R2 set the current through the

voltage divider.

I

V

R2

DIVIDER

REF

=

The power dissipated by the divider resistors is:

P

(R1 R2) I

DIVIDER

DIVIDER

2

=+

×

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 MIC2198

• 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 small-

est MOSFET, which is still adequate for maximum

output current.

• 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 capaci-

tors ESR)

To minimize power loss under heavy loads:

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

tiplying the gate charge by the on-resistance gives

a figure of merit, providing a good balance be-

tween low and high load efficiency.

• Slow transition times and oscillations on the volt-

age 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 resis-

tance. However, using too small of a value will

require more output capacitors to filter the output

ripple, which will force a smaller bandwidth, slower

transient response and possible instability under

certain conditions.

• Lowering the current-sense resistor value will de-

crease the power dissipated in the resistor. How-

ever, it will also increase the overcurrent limit and

will require larger MOSFETs and inductor compo-

nents.

• Use low-ESR input capacitors to minimize the

power dissipated in the capacitors ESR.

Decoupling Capacitor Selection

The 4.7µF decoupling capacitor is used to minimize noise on

the V

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

pins and routed with a wide trace. The capacitor should be a

good quality tantalum. An additional 1µF ceramic capacitor

may be necessary when driving large MOSFETs with high

gate capacitance. Incorrect placement of the V

capacitor will cause jitter or oscillations in the switching

waveform and large variations in the overcurrent limit.

The capacitor should be placed near the IC and connected

directly to between pin 6 (V