Micrel, Inc.

MIC23050

January 2010

9

M9999-012010-D

Applications Information

Input Capacitor

A minimum of 2.2µF ceramic capacitor should be placed

close to the VIN pin and PGND 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 resistive

at high frequencies. This reduces their ability to filter out

high frequency noise.

Output Capacitor

The MIC23050 is designed for use with a 2.2µF or greater

ceramic output capacitor. A low equivalent series

resistance (ESR) ceramic output capacitor either X7R or

X5R is recommended. Y5V and Z5U dielectric capacitors,

aside from the undesirable effect of their wide variation in

capacitance over temperature, become resistive at high

frequencies.

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 MIC23050 is designed for use with an inductance

range from 0.47µH to 2.2µH. Typically, a 1µH inductor is

recommended for a balance of transient response,

efficiency and output ripple. For faster transient response a

0.47µH inductor may be used. For lower output ripple, a

2.2µH is recommended.

Maximum current ratings of the inductor are generally

given in two methods; permissible DC current and

saturation current. Permissible DC current can be rated

either for a 40°C temperature rise or a 10% to 20% loss in

inductance. Ensure the inductor selected can handle the

maximum operating current. When saturation current is

specified, make sure that there is enough margin so that

the peak current of the inductor does not cause it to

saturate. Peak current can be calculated as follows:

As shown by the previous calculation, the peak inductor

current is inversely proportional to the switching frequency

and the inductance; the lower the switching frequency or

the inductance the higher the peak current. As input

voltage increases the peak current also increases.

The size of the inductor depends on the requirements of

the application. Refer to the Application Circuit and Bill of

Material for details.

DC resistance (DCR) is also important. While DCR is

inversely proportional to size, DCR can represent a

significant efficiency loss. Refer to the Efficiency

Considerations.

Compensation

The MIC23050 is designed to be stable with a 0.47µH to

2.2µH inductor with a 2.2µF ceramic (X5R) output

capacitor.

Efficiency Considerations

Efficiency is defined as the amount of useful output power,

divided by the amount of power supplied:

100

I

V

I

V

%

Efficiency

IN

IN

OUT

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 and is critical in hand

held devices.

There are two types of losses in switching converters; DC

losses and switching losses. DC losses are simply the

power dissipation of I

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

Current

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 driving the gates on

and off at a constant 4MHz frequency and the switching

transitions make up the switching losses.