Datasheet

Rfb2 =

Rfb1

- 1

OUT

0.8V

150 k:

- 1

1.2V

0.8V

= 301 k:

Rfb1 =

2SCcf

2S33 pF 32.1 kHz

= 150 k:

Rc =

2SCcf

ESR

2S33 pF 1.7 MHz

= 2.8 k:

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Component Selection

The upper feedback resistor, Rfb1, is selected to provide adequate mid-band gain and to locate a zero at

or below the LC pole frequency. The series resistor, Rc1, is selected to locate a pole at the ESR zero

frequency. Thus:

(14)

Rfb1 and Rc are chosen as 150 kΩ and 2 kΩ in the demo board. With Rfb1 defined based on the voltage

loop requirements, Rfb2, the lower feedback resistor, is then selected for the desired output voltage by:

(15)

Note that Rfb2 has no impact on the control loop from an AC standpoint since the FB pin is the input to an

op-amp type error amplifier and effectively at AC ground. Hence, the control loop can be designed

irrespective of output voltage level. The only caveat here is the necessary derating of the output

capacitance with applied voltage.

The compensation was optimized to work over the full input voltage range. Many applications have a fixed

input voltage rail. It is possible to modify the compensation to obtain a faster transient response for a

given input voltage operating point.

SNVA358B–August 2008–Revised May 2013 AN-1880 LM2854 1MHz Buck Regulator Demo Board

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