MP8720 – 26V, 10A, SYNCHRONOUS BUCK CONVERTER WITH ADJUSTABLE CLM

MP8720 Rev. 1.01

www.MonolithicPower.com

17

8/1/2017

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

APPLICATION INFORMATION

Setting the Output Voltage with No External

Ramp

The

MP8720

does

not

need

ramp

compensation for applications where POSCAP

or ceramic capacitors are set as output

capacitors (when VIN is over 6V), so the

external compensation is not needed. The

output voltage is then set by feedback resistors

R1 and R2 (see Figure 10).

Figure 10: Simplified Circuit without External

Ramp

First, choose a value for R2. R2 should be

chosen reasonably. A small value for R2 leads

to considerable quiescent current loss, but an

R2 value that is too large makes FB noise-

sensitive. It is recommended to choose a value

within 5 - 50kΩ for R2. Use a comparatively

Considering the output ripple, R1 is determined

with Equation (3):

2

1

R

V

V

V

R

REF

REF

OUT







(3)

C4 acts as a feed-forward capacitor to improve

the transient and can be set in the range of

0 - 1000pF. A larger value for C4 leads to better

transient, but it is more noise sensitive.

Reserve room for a noise filter resistor (R9)

(see Figure 11).

Setting the Output Voltage with External

Compensation

If the system is not stable enough or there is

too much jitter when a ceramic capacitor is

and VIN is 5V or lower), an external voltage

ramp should be added to FB through resistor

R4 and capacitor C4. Since there is already an

internal ramp added in the system, a 1MΩ (R4),

220pF (C4) ramp should suffice.

Figure 11: Simplified Circuit with External Ramp

Besides the R1 and R2 divider, the output

voltage is influenced by R4 (see Figure 12). R2

should be chosen reasonably. A small value for

R2 leads to considerable quiescent current loss,

but a value for R2 that is too large makes FB

noise sensitive. It is recommended to choose a

value within 5 - 50kΩ for R2. Use a

high. The value of R1 then is determined with

Equation (4):

2

1

4

2

1

R

R

R

V

V

V

R

REF

OUT

REF









(4)

To get a pole for better noise immunity, set R9

with Equation (5):

9

4SW

1

R

2C

2F







(5)

Set R9 in the range of 100Ω to 1kΩ to reduce

its influence on the ramp.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

a

capacitor to supply AC current to the step-down

converter while maintaining the DC input

voltage. For the best performance, use ceramic

capacitors placed as close to VIN as possible.

Capacitors

with

X5R

and

X7R

ceramic

dielectrics are recommended because they are

fairly stable with temperature fluctuations.

The capacitors must have a ripple current rating

greater than the maximum input ripple current

of the converter. The input ripple current can be

estimated with Equation (6):

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV



 

(6)