MPM3650

– 6A, 17V, 6A, 1.2MHz, STEP-DOWN POWER MODULE WITH DCM

MPM3650 Rev. 1.1

MonolithicPower.com

16

9/17/2021

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

© 2021 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets VOUT. First,

choose a value for R2. Too small of an R2 value

leads to considerable quiescent current (IQ) loss,

while too large an R2 value makes FB noise

sensitive. It is recommended that R2 be between

2

kΩ and 100kΩ. Typically, set the current

flowing through R2 below 250

μA to balance

system stability and minimize load loss. Then R1

can be calculated with Equation (2):

OUT

REF

REF

VV

R1

R2

V





(2)

Figure 5 shows the feedback circuit.

MPM3650

FB

R1

R2

VOUT

RT

CF

Figure 5: Feedback Network

Table 1 shows the recommended resistor values

for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

VOUT (V)

R1

(kΩ)

R2

(kΩ)

CF (pF)

RT

(Ω)

1

20

30

39

0

1.2

20

20

39

0

1.5

20

13

39

0

1.8

20

10

39

0

2.5

20

6.34

39

0

3.3

20

4.42

39

0

Selecting the Input Capacitor (CIN)

The step-down converter has a discontinuous

input current (IIN), and requires a capacitor to

supply the AC current to the converter while

maintaining the DC VIN. Ceramic capacitors are

recommended for the best performance, and

should be placed as close to the VIN pin as

possible. Capacitors with X5R and X7R ceramic

dielectrics are recommended for their stability

amid temperature fluctuations.

The capacitor should have a ripple current rating

greater than the converter

’s maximum input

ripple current. The input ripple current (ICIN) can

be estimated with Equation (3):

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV





 

(3)

The worst-case scenario occurs at VIN = 2 x VOUT,

which can be calculated with Equation (4):

OUT

CIN

I

I

2



(4)

For simplification, choose CIN to have an RMS

current rating greater than half of the maximum

load current (ILOAD).

The

input

capacitance

determines

the

converter’s input voltage ripple (∆VIN). If the

system has an

∆VIN requirement, choose an

input

capacitor

that

meets

the

relevant

specifications.

∆VIN ripple can be estimated with Equation (5):







 



OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

f

C

V

V

(5)

The worst-case scenario occurs at VIN = 2 x VOUT,

which can be calculated with Equation (6):



 



OUT

IN

SW

IN

I

1

V

4

f

C

(6)

Selecting the Output Capacitor (COUT)

The output capacitor (COUT) is required to

maintain the DC VOUT. Ceramic or POSCAP

capacitors

are

recommended.

The

output

voltage ripple (

∆VOUT) can be calculated with

Equation (7):





 











OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

f

L

V

8 f

C

(7)

With

ceramic

capacitors,

the

capacitance

dominates the impedance at fSW, and causes

most of

∆VOUT.

For simplification,

∆VOUT can be calculated with

Equation (8):





 



 

OUT

OUT

OUT

2

SW

OUT

IN

VV

V

(1

)

8 f

L C

V

(8)