MIC4720 datasheet(13/21 Pages) MICREL

13 / 21 page

Micrel, Inc.

MIC4720

May 2007

M9999-051707

Component Selection

Diode Selection

Since the MIC4720 is non-synchronous, a free-wheeling

diode is required for proper operation. A Schottky diode

is recommended due to the low forward voltage drop

and their fast reverse recovery time. The diode should

be rated to be able to handle the average output current.

Also, the reverse voltage rating of the diode should

exceed the maximum input voltage. The lower the

forward voltage drop of the diode the better the

efficiency. Please refer to the layout recommendations to

minimize switching noise.

Input Capacitor

A 10µF ceramic is recommended on each 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. Also, tantalum and electrolytic capacitors

alone are not recommended due their reduced RMS

current handling, reliability, and ESR increases.

An additional 0.1µF 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 to layout recommendations

for proper layout of the input capacitor.

Feedback Resistors

The feedback resistor set the output voltage by dividing

down the output and sending it to the feedback pin. The

feedback voltage is 1.0V. Calculating the set output

voltage is as follows;

Output Capacitor

The MIC4720 is designed for a 4.7µF output capacitor.

X5R or X7R dielectrics are recommended for the output

capacitor. Y5V dielectrics lose most of their capacitance

over temperature and are therefore not recommended.

⎟

⎠

⎞

⎜

⎝

⎛

OUT

Where R1 is the resistor from VOUT to FB and R2 is the

resistor from FB to GND. The recommended feedback

resistor values for common output voltages are available

in the bill of materials on page 19. Although the range of

resistance for the FB resistors is very wide, R1 is

recommended to be 10K. This minimizes the effect the

parasitic capacitance of the FB node.

In addition to a 4.7µF, a small 0.1µF is recommended

close to the load for high frequency filtering. Smaller

case size capacitors are recommended due to there

lower equivalent series ESR and ESL.

The MIC4720 utilizes type III voltage mode internal

compensation

and

utilizes

internal

zero

compensate for the double pole roll off of the LC filter.

For this reason, larger output capacitors can create

instabilities. In cases where a 4.7µF output capacitor is

not sufficient, the MIC4720 offers the ability to externally

control the compensation, allowing for a wide range of

output capacitor types and values.

Feedforward Capacitor (CFF)

A capacitor across the resistor from the output to the

feedback pin (R1) is recommended for most designs.

This capacitor can give a boost to phase margin and

increase the bandwidth for transient response. Also,

large values of feedforward capacitance can slow down

the turn-on characteristics, reducing inrush current. For

maximum phase boost, CFF can be calculated as follows;

Inductor Selection

The MIC4720 is designed for use with a 1µH 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 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.

200kHz

CFF

Bias filter

A small 10Ω resistor is recommended from the input

supply to the bias pin along with a small 0.1µF ceramic

capacitor from bias to ground. This will bypass the high

frequency noise generated by the violent switching of

high currents from reaching the internal reference and

control circuitry. Tantalum and electrolytic capacitors are

not recommended for the bias, these types of capacitors

lose their ability to filter at high frequencies.

MIC4720 Datasheet(PDF) 13 Page - Micrel Semiconductor

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