NB650/NB650H – 6A, 28V, FAST-TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTERS

NB650/NB650H Rev. 1.12

www.MonolithicPower.com

14

10/11/2012

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

© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage-Large ESR Caps

A resistor divider from the output voltage to the

FB pin sets the output voltage. Changing the VID

codes for the NB650/NB650H accomplishes the

same thing.

When there is no external ramp, the output

voltages are set by feedback resistors R1 and

R2A, R2B and R2C. First, choose R1 within 5kΩ-

codes, arranged from low to high. Then

determine R2A, R2B and R2C as follows:

REF

1

OUT1

OUT

REF

2

V

R2A

R1

VV

V

=×

−Δ

−

(14)

1

OUT2

OUT2

REF

2

REF

1

R2B

VV

V

11

VR1 R2A

=

−Δ

−

×−

(15)

1

OUT3

OUT3

REF

2

REF

1

R2C

VV

V

11

VR1 R2A

=

−Δ

−

×−

(16)

REF

1

OUT4

OUT4

2

V

(R1 R2A //R2B //R2C)

VV

R2A //R2B //R2C

×+

=+ Δ

(17)

Where

OUT x

V

Δ

is the output ripple determined by

equation 30.

Setting the Output Voltage-Small ESR Caps

Figure 10: Simplified Ceramic Capacitor Circuit

When using a low-ESR ceramic capacitor on the

output, add an external voltage ramp to FB

through resistor R4 and capacitor C4. The ramp

besides the resistor divider shown in Figure 10.

Choose R1 within 5kΩ-to-100kΩ. The value of

R2 then is determined as follows:

)

V

(V

)

R9

R4

1

R1

1

(

V

R2A

FB(AVG)

OUT1

FB(AVG)

−

×

+

+

=

(18)

R2A

1

R9

R4

1

R1

1

V

V

V

1

R2B

FB(AVG)

FB(AVG)

OUT2

−

+

+

×

−

=

)

(

(19)

R2A

1

R9

R4

1

R1

1

V

V

V

1

R2C

FB(AVG)

FB(AVG)

OUT3

−

+

+

×

−

=

)

(

(20)

17.

value in skip mode is lower than in PWM mode,

which means the load regulation is strictly related

conditions of equation 10 for better load or line

regulation.

following equation:

R9

R1//R2

R1//R2

V

2

1

V

V

RAMP

REF

FB(AVG)

+

+

=

×

(21)

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

following equation 22 for better noise immunity.

Set the value to <(1/5)×R1//R2 to minimize its

SW

2f

C4

2

1

R9

×

×

π

≤

(22)

Using equations 18 through 20 to calculate the

output voltage can be complicated. Furthermore,

accuracy and simplify the R2A, R2B and R2C

to filter the DC influence from R4 and R9. Figure

11 shows a simplified circuit with external ramp

compensation and a DC-blocking capacitor. The

addition of this capacitor simplifies the R2A, R2B

and R2C calculations, as per equations 23-25.