MIC2204

Micrel

M0214-012904

8

January 2004

Applications Information

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 are not recommended: they

lose most of their capacitance over temperature and also

become resistive at high frequencies. This reduces their

ability to filter out high frequency noise.

Output Capacitor

The MIC2204 was designed specifically for the use of a 4.7

µF

ceramic output capacitor. 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 will cause

instability in the MIC2204. For output voltages less than 1.6V,

a 10

µF capacitor may be required for stability. See the

“Compensation” section for more detail.

Total output capacitance should not exceed 15

µF. Large

values of capacitance can cause current limit to engage

during start-up. If larger than 15

µF is required, a feed-forward

capacitor from the output to the feedback node should be

used to slow the start-up time.

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 MIC2204 is designed for use with a 4.7

µH inductor.

Maximum current ratings of the inductor are generally given

in two methods: permissible DC current and saturation cur-

rent. 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 cur-

rent. When saturation current is specified, make sure that

there is enough margin that the peak current will not saturate

the inductor.

The size requirements refer to the area and height require-

ments that are necessary to fit a particular design. Please

refer to the inductor dimensions on their data sheet.

DC resistance is also important. While DCR is inversely

proportional to size, DCR can represent a significant effi-

ciency loss. Refer to the

for a more detailed description.

Table 1 below shows a list of recommended 4.7

µH inductors

by manufacturer, part number and key specifications.

Bias Capacitor

A small 10nF ceramic capacitor is required to bypass the

BIAS pin. The use of low ESR ceramics provides improved

filtering for the bias supply.

Efficiency Considerations

Efficiency is defined as the amount of useful output power,

divided by the amount of power consumed.

Efficiency %

VI

VI

100

OUT

OUT

IN

IN

=

×

×









×

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 handheld devices.

There are two loss terms in switching converters: DC losses

and switching losses. DC losses are simply the power dissi-

side switch during the on cycle, where power loss is equal to

the high-side MOSFET R

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 to drive the gates on and off at a constant

2MHz frequency and the switching transitions make up the

switching losses.

Manufacturer

P/N

H(mm)

W(mm)

L(mm)

DCR(m

Sumida

CDRH2D18-4R7

2

3.2

3.2

81

Murata

LQH43CN4R7M01

2.6

3.2

4.6

150

Murata

LQH32CN4R7M11

2.2

2.7

3.4

195

Coilcraft

1008PS-472M

2.74

3.8

3.8

350

Low Profile

TDK

LDR5610T-4R7MR90

1

5.2

5.8

240

Sumida

CMD4D06

0.8

6.3

5.8

216

Table 1. Component Selection Table