MMM5062 datasheet(10/22 Pages) FREESCALE

10 / 22 page

Micrel, Inc.

MIC22950

February 2010

M9999-021910-B

Application Information

The MIC22950 is a 10A synchronous stepdown

regulator IC with a programmable 400kHz to 2MHz

switching frequency. The control loop is a voltage-mode

PWM control scheme. Other features include tracking

and sequencing control for controlling multiple output

power systems with POR/PG output.

Component Selection

Input Capacitor

A minimum 22µF ceramic capacitor (preferable) is

recommended on each of the PVIN pins for bypassing.

X5R or X7R dielectrics are recommended for the input

capacitor. Do not use Y5V dielectrics, aside from losing

most of their capacitance over temperature and voltage,

they also become resistive at high frequencies. This

reduces their ability to localize high-frequency noise.

Output Capacitor

The MIC22950 was designed specifically for the use of

ceramic output capacitors. It is designed to work with

100µF output capacitor. This output capacitor can be

increased to improve transient performance. Since the

MIC22950 is voltage mode control loop, it 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. Do not use 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 can

cause instability in the MIC22950.

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 MIC22950 is designed for use with a 0.39µH to

2.2µH inductor.

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% loss in

inductance. Ensure that the inductor selected can handle

the maximum operating current. When the saturation

current is specified, make sure that there is enough

margin that the peak current will not saturate the

inductor. The ripple can add as much as 2A to the output

current level. The RMS rating should be chosen to be

equal or greater than the current limit of the MIC22950 to

prevent overheating in a fault condition. For best

electrical performance, the inductor should be placed

very close to the SW nodes of the IC. For this reason,

the heat of the inductor is somewhat coupled to the IC,

which offers some level of protection if the inductor gets

too hot. It is important to test all operating limits before

settling on the final inductor choice.

The size requirements refer to the area and height

requirements 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 typically

inversely proportional to size, DCR can represent a

significant efficiency loss. Refer to the Efficiency

Considerations

section for a more detailed description.

EN/DLY Capacitor

EN/DLY sources 1µA out of the IC to allow a start-up

delay to be implemented. The delay time is simply the

time it takes 1µA to charge CEN/DLY to 1.24V. Therefore:

EN/DLY

DLY

1.24

−

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 mainly two loss terms in switching converters:

Static losses and switching losses. Static losses are

simply the power losses due to V.I or I

2R. For example,

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 RMS Switch Current squared

(ISW

2). During the off cycle, the low-side N-Channel

MOSFET conducts, also dissipating power. Similarly, the

inductor’s DCR and capacitor’s ESR also contribute to

the I

2R losses. Device operating current also reduces

efficiency by the product of the quiescent (operating)

current and the supply voltage. The current required to

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