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AN1317 - APPLICATION NOTE

3.3 Output Capacitor

The output capacitor C5 is an element linked with the desired output ripple amplitude

∆V

depends on the output voltage and on the application to supply.

The worst case occurs for the maximum load, when the VIPer20 delivers its maximum peak current

during the longer conduction time. The charge of the capacitor C5 is:

With the hypothesis that the VIPer20 is at the limit of the continuous mode, which is a worst case making

easier the calculation of the current in the capacitor:

Example with

∆V

The maximum peak current flowing through this capacitor is

during the charge and

during the

discharge. To avoid an excessive power dissipation in the capacitor and a high output ripple, the ESR of

the output capacitor must be low. Table 1 gives a picture of the ESR impact, with

=0.7A typical.

Table 1: Output ripple versus capacitor technology

The above example illustrates that the computed capacitor value has to be tuned according to the

application needs, the capacitor technology and its associated cost.

3.4 VIPer20 Supply Circuit

For both topologies, fig. 7 shows the three different mode:

• The start up phase: The on chip high voltage current source is turned on. It sources a current out of the

then supplies the VIPer20 during the following phase.

Capacitor Type

Capacitor Value

ESR at 100KHz

Output Ripple

Standard Electrolytic

33

µF / 16V

7

Ω

90 mA

4.9 V

Electrolytic Solid Al

33

µF / 16V

700 m

Ω

1460 mA

490 mV

Electrolytic OS-CON

33

µF / 16V

50 m

Ω

1580 mA

35 mV

Electrolytic Low Z

270

µF / 16V

120 m

Ω

630 mA

84 mV

Q

5

it

d

⋅

C

5

∆V

out

⋅

()

=

∫

=

i

⋅

1

2

---

T

s

2

-----

I

p

2

----

⋅





1

8

---T

s

I

p

⋅

==

C

5

1

8

---

T

s

I

⋅

p

∆V

out

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

=

I

p

2

-----

I

p

2

-----

–

I

p