MIC22700 datasheet(11/21 Pages) MICREL

11 / 21 page

Micrel, Inc.

MIC22700

July 2009

M9999-072209-B

Application Information

The MIC22700 is a 7A 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 22µF ceramic is recommended on each of

the PVIN pins 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 MIC22700 was designed specifically for the use of

ceramic output capacitors. 100µF can be increased to

improve transient performance. Since the MIC22700 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 MIC22700.

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 MIC22700 is designed for use with 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 MIC22700 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, so it 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 inversely

proportional to size, DCR can represent a significant

efficiency loss. Refer to the “Efficiency Considerations”

below for a more detailed description.

Enable/DLY Capacitor

Enable/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 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 VI 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

drive the gates on and off at a constant 1MHz or 4MHz

frequency and the switching transitions make up the

switching losses.

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