MP2187 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2187 Rev. 1.0

www.MonolithicPower.com

14

12/12/2014

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APPLICATION INFORMATION

COMPONENT SELECTION

Selecting the Channel 1 Output Voltage

The 1.1V output voltage can be changed with

external feedback resistors. The feedback

resistor R1 and R2 cannot be too large

considering

the

internal

divider

resistors.

Choose R1 +R2 <20kΩ:

OUT1

R1_IN

R2_IN

R1

R2

FB1

MP2187

Using below formulas to set the new output

voltages:

OFB

12

FB

VV

RR

V

−

=×

Where VFB=0.6V.

Table 1 lists the recommended resistors values

for common output voltage.

Table 1: Resistor Values for Common Channel 1

Output Voltages

1.2

3

3

1.5

4.7

3

1.8

6.2

3

2.5

9.53

3

3.3

13.7

3

Selecting the Input Voltage Failure

Threshold

MP2187 can set input voltage failure threshold

with external resistors network. Below is the

input voltage failure circuit:

VIN

R3

R4

PFS

MP2187

R3 is given by:

IN

34

V0.6

RR

0.6

−

=

×

The divider resistors R3 and R4 cannot be too

small considering the leakage from VIN to GND.

Choose R3+R4>1MΩ.

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.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×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 10µF capacitor is sufficient. For higher output

voltage, use 47μF to improve system stability.