AN966 Datasheet, PDF (3/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

Figure 2. Internal Supply Block.
+Vi
AN966 APPLICATION NOTE
8
REF.
-
+
UVLO
D97IN673
ERROR AMPLIFIER AND OVERVOLTAGE DETECTOR BLOCK (see fig. 3 and 4):
The Error Amplifier (E/A) inverting input, through an external divider connected to the output bus, com-
pares a partition of the boosted output DC voltage, Vo, with the internal reference, so as to maintain the
preregulator output DC voltage constant.
The E/A output is used for frequency compensation, usually realised with a feedback capacitor con-
nected to the inverting input. The E/A bandwidth will be extremely low because the output of the E/A
must be constant over a line half-cycle to achieve high PF.
The dynamics of the E/A output is internally clamped so that it can swing between 2V and 5.8V in order
to speed up the recovery after the E/A saturates low due to an overvoltage or saturates high because of
an overcurrent.
The device is provided with a two-level overvoltage protection (OVP), realized by using the pin con-
nected to the E/A output.
In case of overvoltage, the output of the E/A will tend to saturate low but the E/A response is very slow,
so it will take a long time to go into saturation. On the other hand, an overvoltage must be corrected im-
mediately. Hence a fast OVP detector, based on a different concept, is necessary.
In steady state condition, the current through R1 is equal to the current in R2 because the compensation
capacitor does not allow DC current to flow (neither does the high-impedance inverting input of the E/A):
I_R1,R2
=
V_o − 2.5
R1
=
2.5
R2
When the output voltage rises because of a step load change, the current in R1 builds up as well but the
current through R2, fixed by the internal 2.5V reference, does not because of the E/A slowness. The cur-
rent in excess will then flow through the feedback capacitor and enter the low-inpedance E/A output,
where it is sensed. In case, a two-step procedure can occur.
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