L6726A

Application details

17/24

Type II compensation relies on the zero introduced by the output capacitors bank to achieve

stability. Thus, a needed condition to successfully apply type II compensation is

(usually true when output capacitor is based on electrolytic, aluminium electrolytic or

tantalum capacitor).

To define compensation network components values, the below suggestions may be

followed:

a)

Set the output resistor divider in order to obtain the desired output voltage:

(consider trade-off between power dissipation on output resistor divider and offset

introduced by FB bias current).

b)

the approximated formula:

c)

d)

e)

Check that compensation network gain is lower than open loop transconductance

EA gain.

f)

Estimate phase margin obtained (it should be greater than 45°) and repeat,

modifying parameters, if necessary.

8.3

Soft Start time calculation

F

ESR

F

0dB

<

R

FB

R

OS

-----------

V

OUT

V

REF

--------------

1

–

=

R

F

F

0dB

F

ESR

⋅

F

LC

2

----------------------------------

∆V

OSC

V

IN

-------------------

1

gm

--------

R

FB

R

OS

+

R

OS

----------------------------

⋅⋅

⋅

=

C

F

5

2

π R

F

F

LC

⋅⋅

---------------------------------------

=

C

P

C

F

π R

F

C

F

F

SW

1

–

⋅⋅

⋅

------------------------------------------------------------

1

π R

F

F

SW

⋅⋅

------------------------------------

≅

=

t

SS

C

F

V

OUT

V

IN

--------------

∆V

OSC

⋅⋅

I

SS

--------------------------------------------------------

=