Datasheet

2SR

c OUT

c c OUT

0 p FB

f V

R 302 f C

Gain f V

u u u u

PCB Layout

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To select the compensation components, a desired cross over frequency f

should be selected first. It is

recommended to select f

equal to or lower than f

/6. A simplified procedure is given below for R

and C

assuming the capacitor ESR zero is at least three times higher than f

. The compensation resistor can be

found by:

(7)

does not affect the crossover frequency f

, but it sets the compensator zero f

Zcomp

and affects the

phase margin of the loop. For a fast design, C

= 4.7 nF gives adequate performance in most LM21305

applications. Larger C

capacitance gives higher phase margin but at the expese of longer transient

response settling time. It is recommended to set the compensation zero no higher than f

/3 to ensure

enough phase margin, implying:

(8)

For more details, see the LM21305 5A Adjustable Frequency Synchronous Buck Regulator Data Sheet

(SNVS639).

10 PCB Layout

Figure 2. PCB Top Layer

AN-2042 LM21305 Evaluation Board SNVA432C–March 2010–Revised May 2013

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