9/14

AN1621 APPLICATION NOTE

The input capacitor peak current is:

The RMS current that the capacitor has to substain is:

Considering the sensitivity to the temperature of the electrolitic capacitors, and the difficulties in fixing exactly

the effective rms current value, for this particular part number, with an ambient temperature of 40°C, the permit-

ted temperature case is 85 °C.

operating conditions.

2.10 Output capacitors

The output capacitor value is selected on the basis of the output voltage ripple requirement. This ripple is basi-

cally due to the ESR, since the capacitive component is by far lower. Then it must be ensured that the total ESR

is below a maximum value of:

where the ripple voltage has been fixed at 1% max. of Vo.

Three capacitors EKE 1000

µF/35V (ROE) have been paralleled, for a total ESR of about 23mOhm.

2.11 Compensation network

The power supply system can be simplified in two parts: Power and Feedback loop blocks.

Figure 6. Closed Loop Block Diagram

The transfer function of the Power block is:

where:

∆Vo

is the small signal output voltage

∆Vc

is the small signal error amplifier output voltage

I

ch

CV

cp

V

cm

–

()

⋅

t

c

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

440 10

6

–

248

215

–

()

⋅⋅

1.6 10

3

–

⋅

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

8.8A

==

=

I

rms

I

ch

2t

2t

⋅⋅

()

2

–

⋅⋅

⋅

8.8

3.2 10

3

–

50

3.2 10

3

–

50

⋅⋅

()

–

⋅⋅

2

⋅

3.32A

==

=

I

rmstot

I

rms50

2

I

rms200kHz

2

1.6

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

+

3.32

2

2.23

2

+

3.76A

==

=

ESR

max

V

ripple

∆

I

max

∆

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

0.24V

2.6A

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

0.092

Ω

==

=

P

∆V

C

∆V'

O

∆V

O

+

+

-

(2.5V)

D96IN462

G

()

V

o

∆

V

c

∆

----------

n

3R

s

⋅

-------------- R

o

1

S

S

z

------

+

1

S

S

p

------

+

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

⋅⋅

==