Alternative Frequency Compensation Network

A method alternative to the one illustrated in the previous section for cutting down the residual loads is to

physically disconnect the loads by means of series switches. In that case the outputs are actually open.

With this approach, if the application board is repeatedly subjected to quick power-off/power-on cycles

during OFF mode, it may not start-up. In fact, being the load of the 200V output open, after a power off

the output voltage decays very slowly. If the board is powered on again when the output capacitor is still

almost fully charged, the output voltage will rise quickly and overshoot the regulated value. The PWM

may be stopped so long - to allow the output voltage to decay to its correct value - that the L5991 loses

its supply and goes into undervoltage lockout. Next, the L5991 is restarted by R2+R3, the sequence re-

curs and the system gets stuck in this on-off cycle.

To avoid this, it is recommended to use the other feedback configuration provided in the PCB, which

makes use of C20 and R30. As shown in figure A2, in that case C22 and R27 will be omitted and the

value of C21 will be changed. C20 provides an anticipatory effect that prevents the overshoot and the re-

sulting vicious circle above described.

C08

3.3 nF

7

6

1

2

4

3

C19

47

µF 25V

C21

6.8 nF

R28

4.7k

VR1

100k

R29

330k

R30

1.8k

C20

8.2 nF

250V

R26

2.7k

OP1

TPS5904

R16 100

pin 6 of

L5991

R24

47

+15V

out

+200V

out

Figure A2. Alternative compensation network to be used with switch-opened loads. Parts added

or modified are in bold italics.

AN1132 APPLICATION NOTE

11/12