true for low output current where the output

voltage can reach high values, as shown on

figure 11 : 20 V can be reached for a nominal

regulated

one

of

14.5

V,

with

a

typical

transformer. But a simple clamping zener can

limit it to about 17 V with a reasonable dissipated

power. The 10 % to 100 % output load regulation

is better than +/-7 %.

COMPONENTS SIZING

The following procedure defines the value of

essential parameters for the transformer and the

sensing resistance in a typical application. The

user can adapt by himself the final design,

according to specific needs, if any.

V

MAX

OUT

for which the converter has still to

operate in constant current mode.

operating output voltage

V

MIN

OUT

and

V

MAX

OUT

is

lower than 2.5. This ratio is limited by the

overvoltage protection value (Typically 29 V)

and

(Typically

10

V)

and

their

tolerances.

primary to secondary with the formula :

n

=

100

V

MAX

OUT

=

formula :

0.175

from auxiliary to secondary with the formula :

=

25

V

MAX

OUT

=

converter to work in discontinuous mode. The

can be computed by respecting this constraint

in all conditions, or by using the following

formula :

=

n

10

x

V

MIN

IN

x

where :

V

MIN

IN

is the minimum input rectified DC voltage

from the mains.

START UP SEQUENCE

An

integrated high

voltage

current

source

provides a bias current from the DRAIN pin during

the start-up phase. This current is partially

absorbed by internal control circuits which are

placed into a standby mode with reduced

consumption and also provided to the external

soon as the voltage on this pin reaches the high

device

turns

into

active

mode

and

starts

switching. The start up current generator is

switched off, and the converter should normally

provide the needed current on the VDD pin

through the auxiliary winding of the transformer,

as shown on figure 13.

time needed by the converter to start up, when

on many parameters, among which transformer

design,

output

capacitors,

capacitor

value

implemented on the CREF pin (See soft start

consideration here after). The following formula

can be used for defining the minimum capacitor

needed :

>

where :

values.

device begins to switch. Worst case is generally

at full load.

is the voltage hysteresis of the UVLO

logic. Refer to the minimum specified value.

allot a standard 4.7

insures a correct decoupling of the internal serial

Soft start feature is implemented through the

CREF capacitor which is also filtering the CREF

voltage. The minimum value of this capacitor has

to be set according to the switching frequency, in

order to filter the charging and discharging current

issued from the CREF pin (Refer to the current

control description part). It can be increased from

VIPer31SP

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