Application Board Functionality

The outstanding feature of this application board is the so-called Standby Function, directly available

from the L5991. When the power demanded by the load is roughly included between 40 and 90 W (Nor-

mal mode) the switching frequency of the converter is set at 40 kHz. When the monitor enters in low-

consumption mode (Suspend or OFF mode), the power demanded by the load will be much lower, few

Watts. The L5991 will automatically recognise this new operating condition and change the oscillator fre-

and R20.

If the user would like to fine tune the power level that causes the switching frequency to be moved from

sense pin (13, ISEN). This can be accomplished by means of R17, currently not used. The offset will be

the partition of the reference voltage (pin 4, VREF) through R17 and R13.

R10 and R11 allow to provide an additional DC offset on the current sense which depends on the supply

input voltage. This can be used for compensating L5991’s delay to output. In the present case the delay

and R33), which adds a little offset (variable with the duty cycle) on the current sense pin.

Additionally, the board includes some protection functions tipically required, not only in monitor applica-

tions, such as overvoltage (OVP) and overcurrent protection (OCP).

OCP is inherent in the functionality of the L5991: the controller provides both pulse-by-pulse and "hic-

cup" mode current limitation (see Application Information in the datasheet), which fully protect the con-

verter in case of overload or short circuit.

The OVP, in this specific case, is realised by sensing the supply voltage of the L5991 (generated by the

auxiliary winding) through the divider R7-R8 and feeding this partition into pin 14 (DIS). The divider ratio

is such that the OVP is tripped when the supply voltage exceeds 20V. This protection is particularly ef-

fective in case of feedback disconnection.

At maximum load and minimum mains voltage the converter operates at about 55% duty cycle (this is

why slope compensation is required) but no limitation is imposed on its maximum value: L5991’s pin3

(DC) is shorted to pin 4 (VREF). If desired, it is possible to set the maximum duty cycle by adding the di-

vider R34-R35. Please refer to Application Information in L5991 datasheet for calculation of the voltage

divider.

The application board is supplied with a start-up circuit simply made of a dropping resistor (R2+R3) that

draws current from upstream the bridge rectifier.

R05 33K

R04

2.2M

Q01

STK2N50

Q02

BC337

8

R06

10K

4

D04 1N4148

D02

20V

88 to 264

VAC

8

7

12

L5991

11

Figure 3. Low-consumption start-up circuit (not currently implemented)

AN1132 APPLICATION NOTE

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