MIC2128 datasheet(22/32 Pages) MICROCHIP

22 / 32 page

MIC2128

DS20005620A-page 22

 2016 Microchip Technology Inc.

The RMS and saturation current ratings of the selected

inductor should be at least equal to the RMS current

and saturation current calculated in the Equation 5-16

and Equation 5-17.

EQUATION 5-16:

EQUATION 5-17:

Maximizing the efficiency requires the proper selection

of core material and minimizing the winding resistance.

Use of ferrite materials is recommended in the higher

switching frequency applications. Lower cost iron

powder cores may be used but the increase in core

loss reduces the efficiency of the power supply. This is

especially noticeable at low output power. The winding

resistance decreases efficiency at the higher output

current levels. The winding resistance must be

minimized although this usually comes at the expense

of a larger inductor. The power dissipated in the

inductor is equal to the sum of the core and copper

losses. At higher output loads, the core losses are

usually insignificant and can be ignored. At lower

output currents, the core losses can be a significant

contributor. Core loss information is usually available

from the magnetic’s vendor.

The amount of copper loss in the inductor is calculated

by Equation 5-18.

EQUATION 5-18:

5.6

Output Capacitor Selection

The main parameters for selecting the output capacitor

are capacitance value, voltage rating and RMS current

rating. The type of the output capacitor is usually

determined by its equivalent series resistance (ESR).

Recommended capacitor types are ceramic, tantalum,

low-ESR

aluminum

electrolytic,

OS-CON

and

POSCAP. The output capacitor ESR also affects the

control loop from a stability point of view. The maximum

value of ESR can be calculated using Equation 5-19.

EQUATION 5-19:

The required output capacitance to meet steady state

output ripple can be calculated using Equation 5-20.

EQUATION 5-20:

As described in Section 4.1 “Theory of Operation”,

the MIC2128 requires at least 20 mV peak-to-peak

ripple at the FB pin to ensure that the gm amplifier and

the comparator behave properly. Also, the output

voltage ripple should be in phase with the inductor

current. Therefore, the output voltage ripple caused by

the output capacitor’s value should be much smaller

than the ripple caused by the output capacitor ESR. If

low-ESR capacitors, such as ceramic capacitors, are

selected as the output capacitors, a ripple injection

circuit should be used to provide the enough

feedback-voltage ripple. Refer to the Section 5.8

“Ripple Injection”

for details.

The voltage rating of the capacitor should be twice the

output voltage for a tantalum and 20% greater for alu-

minum electrolytic, ceramic or OS-CON. The output

capacitor RMS current is calculated in Equation 5-21.

EQUATION 5-21:

The power dissipated in the output capacitor is shown

in Equation 5-22.

EQUATION 5-22:

5.7

Input Capacitor Selection

The input capacitor reduces peak current drawn from

the power supply and reduces noise and voltage ripple

on the input. The input voltage ripple depends on the

input capacitance and ESR. The input capacitance and

ESR values can be calculated using Equation 5-23.

L_RMS

LOAD(MAX)



I

L_PP



------------------------

Where:

ILOAD(MAX)

= Maximum load current

L_SAT



15mV

DS(ON)

---------------------------------------------

Where:

RCL

= Current limit resistor

ICL

= Current-Limit Source Current (96 µA typical)

RDS (ON) = On-resistance of low-side power MOSFET

INDUCTOR C U



L_RMS



DC R



ESR

V

OUT_PP

I

L_PP

--------------------------



Where:

VOUT_PP

= Peak-to-peak output voltage ripple

IL_PP

= Peak-to-peak inductor current ripple

OUT

I

L_PP





 V

OUT_PP

--------------------------------------------------

Where:

COUT

= Output capacitance value

fSW

= Switching frequency

C_OUT(RMS)

I

L_PP

----------------

DIS(C_OUT)

C_OUT(RMS)

2 ESR

C_OUT



MIC2128 Datasheet(PDF) 22 Page - Microchip Technology

MIC2128 Datasheet(HTML) 22 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL