MIC22600_09 datasheet(12/29 Pages) MICREL

12 / 29 page

Micrel, Inc.

MIC22600

October 2009

M9999-102809-C

Application Information

The MIC22600 is a 6A Synchronous step down regulator

IC with a fixed 1 MHz, voltage mode PWM control

scheme. The other features include tracking and

sequencing control for controlling multiple output power

systems, power on reset.

Component Selection

Input Capacitor

A minimum 10µF ceramic is recommended on each of

the PVIN pins for bypassing. X5R or X7R dielectrics are

recommended for the input capacitor. Y5V dielectrics is

not recommended.

Output Capacitor

The MIC22600 was designed specifically for the use of

ceramic output capacitors and 22µF is optimum output

capacitor. 22µF can be increased to 100µF to improve

transient performance. Since the MIC22600 is in voltage

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 can cause instability in the MIC22600.

Inductor Selection

Inductor selection is determined by the following (not

necessarily in the order of importance):

•

Inductance

•

Rated current value

•

Size requirements

•

DC resistance (DCR)

The MIC22600 is designed to use a 0.47µH to 4.7µH

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 the inductor selected can handle the

maximum operating current. When 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 1.2A to the output current level. The

RMS rating should be chosen to be equal or greater than

the Current Limit of the MIC22600 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. 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.

DCR is inversely proportional to size and can represent

a significant efficiency loss. Refer to the “Efficiency

Considerations” below for a more detailed description.

EN/DLY Capacitor

EN/DLY sources 1µA out of the IC to allow a startup

delay to be implemented. The delay time is simply the

time it takes 1µA to charge CDLY to 1.25V. Therefore:

−

⋅

DLY

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

decreases power dissipation in the power supply,

reducing the need for heat sinks and thermal design

considerations and it decreases consumption of current

for battery powered applications. Reduced current drawn

from a battery increases the devices operating time,

particularly in hand held devices.

There are mainly two loss terms in switching converters:

conduction losses and switching losses. Conduction

losses are simply the power losses due to VI or I

2R. For

example, power is dissipated in the high side switch

during the on cycle. The 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 power

consumed at 1MHz frequency and power loss due to

switching transitions add up to switching losses.

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