MPM3686 – 18V 20A STEP-DOWN POWER MODULE

MPM3686 Rev. 1.02

www.MonolithicPower.com

16

10/13/2016

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APPLICATION INFORMATION

Setting

the

Output

Voltage-Small

ESR

Capacitors

When the output capacitors are all ceramic

capacitors or capacitors with small ESR, external

RAMP is injected through the R/C network

across the inductor. The circuit connection is as

follows:

Figure 8

Here is the procedure to find the values for Ctrim

and Rtrim:

a) Determine the Ton

FREQ

ON

IN

5.3 R

(K )

T(ns)

V(V) 0.4







(10)

10mV-30mV for most of the applications.

)

ns

(

T

)

V

(

V

)

K

(

R

)

V

(

V

)

V

(

V

)

pF

(

C

ON

RAMP

OUT

IN

TRIM











3

(11)

RAMP

FB _ AVG

REF

V(V)

V(V) V

(V)

2



(12)

d) Calculate Rtrim to get the desired output

voltage:

)

K

(

)

)

V

(

V

)

V

(

V

(

)

K

(

R

AVG

_

FB

OUT

o











10

1

(13)

O

TRIM

O

R (K ) 300(K )

R(K )

300(K ) R (K )











(14)

Setting

the

Output

Voltage-Large

ESR

Capacitors

If one or more piece of the output capacitors

have large ESR, then there is no need of external

RAMP. Otherwise, it’ll generate group pulses at

light

load

conditions.

The

Ctrim

is

still

recommended to boost the phase margin of the

system. A value between 100pF and 2.2nF is

recommended. The circuit connection can be

made as the following:

Figure 9

The Rtrim can be determined as follows to obtain

the desired output voltage:

OUT

REF

TRIM

REF

VV

R(K )

10

V



 

(15)

Input Capacitor

The input current to the step-down power module

is discontinuous, and therefore, it requires a

capacitor to supply the AC current to the step-

down power module while maintaining the DC

input voltage. Use ceramic capacitors for best

performance. During layout, Place the input

capacitors as close to the IN and GND pins as

possible.

The capacitance can vary significantly with

temperature. Use capacitors with X5R or X7R

ceramic dielectrics because they are fairly stable

over a wide temperature range.

The capacitors must also have a ripple current

rating that exceeds the converter’s maximum

input ripple current. Estimate the input ripple

current as follows:

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

OUT

CIN









(16)

where:

2

I

I

OUT

(17)