MIC2205 datasheet(13/16 Pages) MICREL

13 / 16 page

Micrel, Inc.

MIC2205

April 2005

M9999-041105

www.micrel.com

Applications Information

The MIC2205 is a 600mA PWM power supply that

utilizes a LOWQ™ light load mode to maximize

battery efficiency in light load conditions. This is

achieved with a LOWQ control pin that when pulled

low, shuts down all the biasing and drive current for

the PWM regulator, drawing only 18

µA of operating

current. This allows the output to be regulated

through the LDO output, capable of providing 60mA

of output current. This method has the advantage of

producing a clean, low current, ultra low noise output

in LOWQ™ mode. During LOWQ™ mode, the SW

node becomes high impedance, blocking current

flow. Other methods of reducing quiescent current,

such as pulse frequency modulation (PFM) or

bursting techniques, create large amplitude, low

frequency ripple voltages that can be detrimental to

system operation.

When more than 60mA is required, the LOWQ pin

can be forced high, causing the MIC2205 to enter

PWM mode. In this case, the LDO output makes a

"hand-off" to the PWM regulator with virtually no

variation in output voltage. The LDO output then

turns off allowing up to 600mA of current to be

efficiently supplied through the PWM output to the

load.

Input Capacitor

A minimum 1

µF ceramic is recommended on the

VIN pin for bypassing. X5R or X7R dielectrics are

recommended

for

the

input

capacitor.

Y5V

dielectrics lose most of their capacitance over

temperature and are therefore, not recommended.

A minimum 1

µF is recommended close to the VIN

and PGND pins for high frequency filtering. Smaller

case size capacitors are recommended due to their

lower

ESR

and

ESL. Please

refer

layout

recommendations for proper layout of the input

capacitor.

Output Capacitor

Even though the MIC2205 is optimized for a 2.2

µF

output capacitor, output capacitance can be varied

from 1

µF to 4.7µF. The MIC2205 utilizes type III

internal compensation and utilizes an internal high

frequency zero to compensate for the double pole

roll off of the LC filter. For this reason, larger output

capacitors can create instabilities. X5R or X7R

dielectrics

are

recommended

for

the

output

capacitor.

Y5V

dielectrics

lose

most

their

capacitance over temperature and are therefore, not

recommended.

In addition to a 2.2

µF, a small 10nF is recommended

close to the load for high frequency filtering. Smaller

case size capacitors are recommended due to there

lower ESR and ESL.

Inductor Selection

The MIC2205 is designed for use with a 2.2

µH

inductor.

Proper

selection

should

ensure

the

inductor can handle the maximum average and peak

currents required by the load. 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 that 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. Peak inductor current can be

calculated as follows:

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

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