MP8756 – 26V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8756 Rev. 1.1

www.MonolithicPower.com

15

4/13/2017

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage with an E-

Capacitor or POS Capacitor

For applications that use an electrolytic capacitor

or POS capacitor with a controlled ESR output is

set

as

an

output

capacitor,

external

compensation is not need. The output voltage is

set by feedback resistors R1 and R2 (see Figure

6).

R1

R2

ESR

POSCAP

SW

FB

Vo

L

Figure 6: Simplified Circuit of POS Capacitor

The value for R2 must be chosen carefully since

a small R2 value leads to considerable quiescent

current loss, while a value that is too large makes

FB noise sensitive. R2 is recommended to be

within 5kΩ - 100kΩ. Typically, set the current

through R2 between 5µA - 30µA to create a good

balance between the system stability and no-load

loss. Considering the output ripple, calculate R1

with Equation (3):

OUT

REF

REF

VV

R1

R2

V





(3)

Setting the Output Voltage with a Pure

Ceramic Output Capacitor

The

MP8756

employs

internal

ramp

compensation. When the internal compensation

is enough for stable operation with the ceramic

output capacitors, the external resistor divider is

Then R1 can be determined with Equation (4):

OUT

REF

REF

VV

R1

R2

V





(4)

The feedback circuit is shown in Figure 7.

Figure 7: Feedback Network

When the internal ramp compensation is not

enough to stabilize the loop with a pure ceramic

capacitor, an extra external voltage ramp around

20mV should be added to FB through resistor R4

and capacitor C5.

Figure 8: External Ramp Compensation

Figure 8 shows a simplified external ramp

be estimated with Equation (5):







in

out

ramp

on

45

VV

VT

RC

(5)

For better load or line regulation, use a lower

around 20mV.

The MP8756 employs a DC auto-tune loop to

can maintain 0.6V, even with an external ramp

compensation circuit. Figure 9 shows the DC

equivalent circuit with an external ramp circuit.

Figure 9: Equivalent DC Circuit