MP2188 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2188 Rev. 1.01

www.MonolithicPower.com

13

2/2/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The MP2188 output voltage of the two channels

can be changed with external feedback

resistors. The feedback resistors cannot be too

large considering the internal divider resistors.

Choose external feedback resistors <20kΩ:

Using below formulas to set the new output

voltages:

R1_IN

V

V

I

CH

OUT

FB

−

=

R

_

ESD

I

6

.

0

V

FB

FB

×

−

=

2

2

FB

FB

FB

OUT

1

R

R

I

V

V

V

R

×

×

−

−

=

each channel.

ESD_R=3.59kOhm.

ESD_R=4.28kOhm.

To mask internal resistor well, the adjustable

Table 1 lists the recommended resistors values

for common output voltage.

Table 1: Resistor Values for Common Output

Voltages

1.1V output channel

1.8V output channel

R1(kΩ)

R2(kΩ)

R3(kΩ)

R4(kΩ)

1.2

3.01

3

3

3

1.5

4.7

3

4.3

3

1.8

6.34

3

6.2

3

2.5

10.5

3

10

3

3.3

15.4

3

14.7

3

Selecting the Inductor

A 0.82µH to 4.7µH inductor is recommended for

most applications. For highest efficiency, chose

an inductor with a DC resistance less than

15mΩ. For most designs, the inductance value

can be derived from the following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

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

Δ

=+

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, use 47μF 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:

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

CIN

V

V

1

V

V

I

I

The worse case condition occurs at VIN =

2

I

I

LOAD

For simplification, choose an input capacitor

whose RMS current rating is greater than half of

the maximum load current.