MP8758–18V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758 Rev. 1.0

www.MonolithicPower.com

16

1/13/2015

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APPLICATION INFORMATION

Setting the Output Voltage---without external

compensation

The MP8758 can usually support different type of

output capacitors, including POSCAP, electrolytic

capacitor and also ceramic capacitors without

external ramp compensation. The output voltage

is then set by feedback resistors R1 and R2. As

Figure 10 shows.

R1

R2

CAP

SW

FB

Vo

L

C4

Figure10—Simplified Circuit of POS Capacitor

First, choose a value for R2. R2 should be

chosen reasonably, a small R2 will lead to

considerable quiescent current loss while too

large R2 makes the FB noise sensitive. Typically,

set the current through R2 at around 5-10uA will

make a good balance between system stability

and also the no load loss. Then R1 is determined

as follow with the output ripple considered:

OUT

OUT

REF

12

REF

1

VV

V

2

RR

V

−Δ

−

=⋅

(13)

OUT

V

Δ

is the output ripple, refer to equation (23).

Other than feedback resistors, a feed forward

cap C4 is usually applied for a better transient

performance, especially when ceramic caps are

applied for their small capacitance, a cap value

around 100pF-1nF is suggested for a better

transient while also keep the system stable with

enough noise immunity. In case the system is

noise sensitive because of the zero induced by

this cap, add a resistor-usually named as R9

between this cap and FB to form a pole, this

resistor can be set according to equation (16) as

in the following section.

Setting the Output Voltage ―with external

compensation

R1

R2

Ceramic

SW

FB

Vo

L

R9

R4

C4

Figure11—Simplified Circuit of Ceramic Capacitor

If the system is not stable enough when low ESR

ceramic capacitor is used in the output, an

external voltage ramp should be added to FB

through resistor R4 and capacitor C4.

The output voltage is influenced by ramp voltage

equation (5). R2 should be chosen reasonably, a

small R2 will lead to considerable quiescent

current loss while too large R2 makes the FB

noise sensitive. It is recommended to choose a

value

within 5kΩ-50kΩ for

R2,

using

a

comparatively larger R2 when Vo is low, etc.,

1.05V, and a smaller R2 when Vo is high. And

the value of R1 then is determined as follow:

2

1

FB(AVG)

2

OUT

FB(AVG)

4

9

R

R=

V

R

-

(V

-V

) R +R

(14)

condition, etc., its value on the skip mode would

be lower than that of the PWM mode, which

means the load regulation is strictly related to the

regulation, a lower Vramp is suggested, as long

as the criterion shown in equation (7) can be met.

deduced from the equation below.

12

FB(AVG)

REF

RAMP

12

9

R//R

1

VV

V

2R //R

R

=+

×

+

(15)