which gives the normal operating frequency, and:

≈

1

(2)

which gives the standby operating frequency, that is the one the converter will operate at when lightly

(3),

= 



= VREF

= GND/OPEN

(4)

is related to the duration of the falling-edge of the sawtooth. In case V15 is connected to VREF, how-

ever, the switching frequency will be a half the values resulting from (1) and (2).

Fig.3 shows also the comparator with hysteresis that recognizes the load condition of the converter. The

to Fig. A1 in Appendix, the peak voltages on the current sense pin of the L5991 (ISEN, 13) relevant to

=

3

=

2.5

− 2 ⋅ 0.7

3

= 0.367V

(5)

=

3

=

4.0

− 2 ⋅ 0.7

3

= 0.867V

(6).

can be immediately related to the peak input current. Although having fixed thresholds may seem a lack

of flexibility, in reality it is possible to adjust the thresholds in terms of input power level, if needed, by

Standby Operation Analysis

In this context, flyback converters are classified as stated in the appendix "Flyback Basics". Another as-

sumption is that the delay to output of the L5991 is compensated, thus the offset voltage Vo is intended

as the amount exceeding the value needed for compensation (see Appendix "Peak current mode control

Basics"). This analysis does not take other non-idealities into consideration, thus the results are approxi-

mate.

Please refer to the appendix for an explanation of symbols, terminology and formulas.

=

=

0.367

(7),

corresponding to the standby input power which, under the assumption of DCM (Discontinuous Conduc-

tion Mode) operation, can be expressed as:

=

1

2







0.367







2

(8).

=

1

√

2

(9).

AN1049 APPLICATION NOTE

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