MIC2202 datasheet(7/18 Pages) MICREL

7 / 18 page

May 2004

M9999-052104

MIC2202

Micrel

Applications Information

Input Capacitor

A minimum 1

µF ceramic is recommended on the V

IN pin for

bypassing. X5R or X7R dielectrics are recommended for the

input capacitor. Y5V dielectrics, aside from losing most of

their capacitance over temperature, they also become resis-

tive at high frequencies. This reduces their ability to filter out

high frequency noise.

Output Capacitor

The MIC2202 was designed specifically for the use of a 1

µF

ceramic output capacitor. This value can be increased to

improve transient performance. Since the MIC2202 is volt-

age mode, the control loop relies on the inductor and output

capacitor for compensation. For this reason, do not use

excessively large output capacitors. The output capacitor

requires either an X7R or X5R dielectric. Y5V and Z5U

dielectric capacitors, aside from the undesirable effect of their

wide variation in capacitance over temperature, become

resistive at high frequencies. Using Y5V or Z5U capacitors

will cause instability in the MIC2202.

Total output capacitance should not exceed 15

µF. Large

values of capacitance can cause current limit to engage

during start-up. If larger than 15

µF is required, a feed-forward

capacitor from the output to the feedback node should be

used to slow the start up time.

Inductor Selection

Inductor selection will be determined by the following (not

necessarily in the order of importance):

• Inductance

• Rated current value

• Size requirements

• DC resistance (DCR)

The MIC2202 is designed for use with a 1

µH to 4.7µH

inductor.

Maximum current ratings of the inductor are generally given

in two methods: permissible DC current and saturation cur-

rent. Permissible DC current can be rated either for a 40

°C

temperature rise or a 10% loss in inductance. Ensure the

inductor selected can handle the maximum operating cur-

rent. When saturation current is specified, make sure that

there is enough margin that the peak current will not saturate

the inductor.

The size requirements refer to the area and height require-

ments that are necessary to fit a particular design. Please

refer to the inductor dimensions on their datasheet.

DC resistance is also important. While DCR is inversely

proportional to size, DCR can represent a significant effi-

ciency loss. Refer to the

“Efficiency Considerations” below

for a more detailed description.

Bias Capacitor

A small 10nF ceramic capacitor is required to bypass the bias

pin. The use of low ESR ceramics provides improved filtering

for the bias supply.

Efficiency Considerations

Efficiency is defined as the amount of useful output power,

divided by the amount of power consumed.

Efficiency %

100

OUT









Maintaining high efficiency serves two purposes. It reduces

power dissipation in the power supply, reducing the need for

heat sinks and thermal design considerations and it reduces

consumption of current for battery powered applications.

Reduced current draw from a battery increases the devices

operating time, critical in hand held devices.

There are two loss terms in switching converters: DC losses

and switching losses. DC losses are simply the power dissi-

pation of I2R. Power is dissipated in the high side switch

during the on cycle. Power loss is equal to the high side

MOSFET RDS

(ON) multiplied by the Switch Current

2. During

the off cycle, the low side N-Channel MOSFET conducts, also

dissipating power. Device operating current also reduces

efficiency. The product of the quiescent (operating) current

and the supply voltage is another DC loss. The current

required to drive the gates on and off at a constant 2MHz

frequency and the switching transitions make up the switch-

ing losses.

Figure 2 shows an efficiency curve. The non-shaded portion,

from 0mA to 200mA, efficiency losses are dominated by

quiescent current losses, gate drive and transition losses. In

this case, lower supply voltages yield greater efficiency in that

they require less current to drive the MOSFETs and have

reduced input power consumption.

100

0.1

0.2

0.3

0.4

0.5

0.6

OUTPUT CURRENT (A)

3.3V

OUT

4.2V

Efficiency

vs. Output Current

Figure 2. Efficiency Curve

The shaded region, 200mA to 500mA, efficiency loss is

dominated by MOSFET RDS

(ON) and inductor DC losses.

Higher input supply voltages will increase the Gate-to-Source

threshold on the internal MOSFETs, reducing the internal

RDS

(ON). This improves efficiency by reducing DC losses in

the device. All but the inductor losses are inherent to the

device. In which case, inductor selection becomes increas-

ingly critical in efficiency calculations. As the inductors are

reduced in size, the DC resistance (DCR) can become quite

significant. The DCR losses can be calculated as follows;

DCR

OUT

=×

From that, the loss in efficiency due to inductor resistance can

be calculated as follows:

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