MIC4930 datasheet(14/24 Pages) MICROCHIP

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MIC4930

DS20005669A-page 14

 2016 Microchip Technology Inc.

5.0

APPLICATION INFORMATION

The MIC4930 is a highly efficient, 3A synchronous

buck regulator ideally suited for supplying processor

core and I/O voltages from a 5V or 3.3V bus.

5.1

Input Capacitor

A 10 μF ceramic capacitor or greater should be placed

close to the PVIN pin and PGND pin for bypassing. A

X5R or X7R temperature rating is recommended for the

input capacitor. Take into account C vs. bias effect in

order to estimate the effective capacitance and the

input ripple at the VIN voltage.

5.2

Output Capacitor

The MIC4930 is designed for use with a 10 μF or

greater ceramic output capacitor. Increasing the output

capacitance will lower output ripple and improve load

transient response. A low equivalent series resistance

(ESR) ceramic output capacitor is recommended

based upon performance, size, and cost. Ceramic

capacitors with X5R or X7R temperature ratings are

recommended.

5.3

Inductor Selection

When selecting an inductor, it is important to consider

the following factors:

• Inductance

• Rated current value

• Size requirements

• DC resistance (DCR)

• Core losses

The MIC4930 is designed for use with a 1 μH to 2.2 μH

inductor. For faster transient response, a 1 μH inductor

will yield the best result. For lower output ripple, a 2.2

μH inductor is recommended.

Inductor current ratings are generally given in two

methods: permissible DC current, and saturation

current. Permissible DC current can be rated for a 20°C

to 40°C temperature rise. Saturation current can be

rated for a 10% to 30% loss in inductance. Ensure that

the nominal current of the application is well within the

permissible DC current ratings of the inductor, also

depending on the allowed temperature rise. Note that

the inductor permissible DC current rating typically

does not include inductor core losses. These are a very

important contribution to the total inductor core loss

and temperature increase in high-frequency DC-to-DC

converters, since core losses increase with at least the

square of the excitation frequency. For more accurate

core loss estimation, it is recommended to refer to

manufacturers’ datasheets or websites.

When saturation current is specified, make sure that

there is enough design margin, so that the peak current

does not cause the inductor to enter saturation.

Also pay attention to the inductor saturation

characteristic in current limit. The inductor should not

heavily saturate even in current limit operation,

otherwise the current might instantaneously run away

and reach potentially destructive levels. Typically,

ferrite-core inductors exhibit an abrupt saturation

characteristic, while powdered-iron or composite

inductors have a soft-saturation characteristic.

Peak current can be calculated by using Equation 5-1.

EQUATION 5-1:

As shown by the calculation above, the peak inductor

current is inversely proportional to the switching

frequency and the inductance. The lower the switching

frequency or inductance, the higher the peak current.

As input voltage increases, the peak current also

increases.

The size of the inductor depends on the requirements

of the application. Refer to the typical application circuit

and Bill of Materials for details.

DC resistance (DCR) is also important. While DCR is

inversely proportional to size, DCR can represent a

significant efficiency loss. Refer to the Efficiency

Considerations subsection.

5.4

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied. (See

Typical Performance Curves section).

EQUATION 5-2:

There are two types of losses in switching converters;

DC losses and switching losses. DC losses are simply

the power dissipation of I2R. Power is dissipated in the

high side switch during the on cycle. Power loss is

equal to the high side MOSFET RDSON multiplied by

the switch current squared. During the off cycle, the low

side N-channel MOSFET conducts, also dissipating

power. The device operating current also reduces

efficiency. The product of the quiescent (operating)

current and the supply voltage represents another DC

loss. The current required driving the gates on and off

at high frequency and the switching transitions make

up the switching losses.

At the higher currents for which the MIC4930 is

designed, efficiency loss is dominated by MOSFET

RDSON and inductor losses. Higher input supply

voltages will increase the gate-to-source threshold on

the internal MOSFETs, thereby reducing the internal

PEAK

OUT

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–

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Efficiency%

OUT

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100

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