MIC2875 datasheet(14/24 Pages) MICROCHIP

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MIC2875

DS20005549A-page 14

 2015 Microchip Technology Inc.

6.0

COMPONENT SELECTION

6.1

Inductor

Inductor selection is a trade-off between efficiency,

stability, cost, size, and rated current. Because the

boost converter is compensated internally, the

recommended inductance is limited from 1 μH to

2.2 μH to ensure system stability and presents a good

balance between these considerations.

A large inductance value reduces the peak-to-peak

inductor ripple current hence the output ripple voltage.

This also reduces both the DC loss and the transition

loss at the same inductor’s DC resistance (DCR).

However, the DCR of an inductor usually increases

with the inductance in the same package size. This is

due to the longer windings required for an increase in

inductance. Since the majority of the input current

passes through the inductor, the higher the DCR the

lower the efficiency is, and more significantly at higher

load currents. On the other hand, inductor with smaller

DCR but the same inductance usually has a larger size.

The saturation current rating of the selected inductor

must be higher than the maximum peak inductor

current to be encountered and should be at least 20%

to 30% higher than the average inductor current at

maximum output current.

6.2

Input Capacitor to the Device

Supply

A ceramic capacitor of 1 μF or larger with low ESR is

recommended to reduce the input voltage ripple to

ensure a clean supply voltage for the device. The input

capacitor should be placed as close as possible to the

MIC2875 IN pin and AGND pin with short traces to

ensure good noise performance. X5R or X7R type

ceramic capacitors are recommended for better

tolerance over temperature. The Y5V and Z5U type

temperature rating ceramic capacitors are not

recommended due to their large reduction in

capacitance

over

temperature

and

increased

resistance at high frequencies. The use of these

reduces the ability to filter out high-frequency noise.

The rated voltage of the input capacitor should be at

least 20% higher than the maximum operating input

voltage over the operating temperature range.

6.3

Input Capacitor to the Power Path

A ceramic capacitor of a 4.7 μF of larger with low ESR

is recommended to reduce the input voltage fluctuation

at the voltage supply of the high current power path. An

input capacitor should be placed close to the VIN supply

to the power inductor and PGND for good device

performance at heavy load condition. X5R or X7R type

ceramic capacitors are recommended for better

tolerance overtemperature.

The Y5V and Z5U type temperature rating ceramic

capacitors are not recommended due to their large

reduction in capacitance over temperature and

increased resistance at high frequencies. These

reduce their ability to filter out high-frequency noise.

The rated voltage of the input capacitor should be at

least 20% higher than the maximum operating input

voltage over the operating temperature range.

6.4

Output Capacitor

Output capacitor selection is also a trade-off between

performance, size, and cost. Increasing output

capacitor will lead to an improved transient response,

however, the size and cost also increase. For operation

with VIN ≤ 5.0V, a minimum of 22 μF output capacitor

with ESR less than 10 mΩ is required. For operation

with VIN > 5.0V, a minimum of 66 μF output capacitor

with ESR less than 10 mΩ is required. X5R or X7R type

ceramic capacitors are recommended for better

tolerance over temperature. Additional capacitors can

be added to improve the transient response, and to

reduce the ripple of the output when the MIC2875

operates in and out of bypass mode.

The Y5V and Z5U type ceramic capacitors are not

recommended due to their wide variation in

capacitance

over

temperature

and

increased

resistance at high frequencies. The rated voltage of the

output capacitor should be at least 20% higher than the

maximum operating output voltage over the operating

temperature range. 0805 size ceramic capacitor is

recommended for smaller ESL at output capacitor

which contributes smaller voltage spike at the output

voltage of high-frequency switching boost converter.

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