MP8764 ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER WITH LATCH-OFF OCP

MP8764 Rev. 1.2

www.MonolithicPower.com

18

6/21/2016

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APPLICATION INFORMATION

Setting the Output Voltage-Large ESR Caps

For applications that electrolytic capacitor or POS

capacitor with a controlled output of ESR is set

as output capacitors. The output voltage is set by

feedback resistors R1 and R2. As figure 10

shows.

R1

R2

ESR

POSCAP

SW

L

FB

Figure 10—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. It is

recommended to choose a value within 5k -

50k

for R2, using a comparatively larger R2

high. Then R1 is determined as follow with the

output ripple considered:

OUT

OUT

REF

REF

1

V

V

V

2

R1

R2

V

−

× ∆

−

=

×

(15)

OUT

V

∆

is the output ripple determined by equation

24.

Setting the Output Voltage-Small ESR Caps

R1

R2

Ceramic

SW

FB

L

R9

R4

C4

Figure 11—Simplified Circuit of Ceramic

Capacitor

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

shown in equation 7. 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 5k -50k

for R2, using a

R1 then is determined as follow:

9

R

4

R

2

R

V

V

V

2

R

1

R

)

AVG

(

FB

OUT

)

AVG

(

FB

+

−

−

=

(16)

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

meets equation 9.

deduced from equation 17.

RAMP

REF

AVG

FB

V

V

V

×

+

=

2

1

)

(

(17)

Usually, R9 is set to 0 , and it can also be set

following equation 18 for a better noise immunity.

It also should be set to be 5 timers smaller than

1

R1 R2

R9

5

R1 R2

×

<

×

+

(18)

Using equation 16 and 17 to calculate the output

voltage can be complicated. To simplify the

calculation of R1 in equation 11, a DC-blocking

influence from R4 and R9. Figure 12 shows a

simplified

circuit

with

external

ramp

compensation and a DC-blocking capacitor. With

this capacitor, R1 can easily be obtained by

using equation 19 for PWM mode operation.