MIC4782 datasheet(13/23 Pages) MICREL

13 / 23 page

Micrel, Inc.

MIC4782

August 2009

M9999-081709-D

Component Selection

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 to 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 recommendation

for proper layout of the input capacitor.

Output Capacitor

The MIC4782 is designed to be stable with a 4.7µF

output

capacitor.

X5R

X7R

dielectrics

are

recommended for the output capacitor. Y5V dielectrics

lose most of their capacitance over temperature and are

therefore not recommended.

In addition to a 4.7µF or larger value output capacitor, 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 MIC4782 utilizes type III voltage mode internal

compensation

and

utilizes

internal

zero

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

Inductor Selection

The MIC4782 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.

Diode Selection

Since the MIC4782 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 recommendation to

minimize switching noise.

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 0.6V. Calculating the set output

voltage is as follows;

⎟

⎠

⎞

⎜

⎝

⎛

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 of this data sheet.

Although the range of resistance for the FB resistors is

very wide, R1 is recommended to be 10KΩ. This

minimizes the parasitic capacitance effect of the FB

node.

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;

200kHz

CFF

Large values of feedforward capacitance may introduce

negative FB pin voltage during load shorting, which will

cause latch-off. In that case, a Schottky diode from FB

pin to the ground is recommended.

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.

Voltage Derating of Ceramic Capacitors

The capacitance of ceramic capacitors drops at high

voltage. Figure 7 shows typical voltage derating curves

of X5R 6.3V ceramic capacitors. At half of the rating

voltage and room temperature, the capacitance of 0603

X5R capacitors can drop about 30%, while the 0805

package only drops by 5%. Therefore, 0805 package

ceramic capacitors are preferred if the application

voltage is close to half of the capacitor rating voltage or

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