MP2145

– 5.5V, 6A, 1.2MHz, COT, BUCK SWITCHER

MP2145 Rev. 1.11

www.MonolithicPower.com

16

12/20/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application schematic).

Select

the

feedback

resistor

R1

which

considers reducing VOUT leakage current,

typically between 40k

Ω to 200kΩ. There is not

strict

requirement

on

feedback

resistor.

R1>10kΩ is reasoned for some application. R2

can be gotten below then:

out

R1

R2

V

1

0.6





The feedback circuit is shown as Figure 6:

R1

R2

Vout

FB

MP2145

Figure 6: Feedback Network

Table 1 lists the recommended resistors values

for common output voltages.

Table 1: Resistor Values for Common Output

Voltages

R1 (k

Ω)

R2 (k

Ω)

1.0

200(1%)

300(1%)

1.2

200(1%)

200(1%)

1.8

200(1%)

100(1%)

2.5

200(1%)

63.2(1%)

3.3

200(1%)

44.2(1%)

Selecting the Inductor

In order to achieve high efficiency at light load,

a low value inductor such as 0.47 µH is

recommended

for

most

applications.

For

highest efficiency, chose an inductor with a DC

resistance less than 30m

Ω. For most designs,

the inductance value can be derived from the

following equation.

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

Where

ΔI

Choose an inductor current to be approximately

30%

of

the

maximum

load

current.

The

maximum inductor peak current is:

L

L(MAX)

LOAD

I

II

2





Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, and requires a capacitor to

supply

the

AC

current

to

the

step-down

converter

while

maintaining

the

DC

input

voltage. Use low-ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R

dielectrics

are

highly

recommended

because of their low ESR values and small

temperature coefficients. For most applications,

a 22µF capacitor is sufficient. For higher output

voltage, 47uF may be needed to improve

system stability.

Since the input capacitor absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

OUT

OUT

C1

LOAD

IN

IN

VV

I

I

1

VV

















The worse case condition occurs at VIN =

LOAD

C1

I

I

2



For simplification, choose an input capacitor

whose RMS current rating greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, use a small high-quality ceramic

capacitor (0.1μF), placed as close to the IC as

possible. When using ceramic capacitors, make

sure that they have enough capacitance to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V









 





