DAP005 datasheet(16/39 Pages) STMICROELECTRONICS

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Application information

DAP005

16/39

Application information

6.1

Overvoltage protection

Normally, the voltage control loop keeps the output voltage VO of the PFC pre-regulator

close to its nominal value, set by the ratio of the resistors R1 and R2 of the output divider.

Neglecting the ripple components, under steady state conditions the current through R1

equals that through R2. Considering that the non-inverting input of the error amplifier is

internally biased at 2.5V, the voltage at pin INV will be 2.5V as well, then:

Equation 1

If the output voltage experiences an abrupt change

∆Vo the voltage at pin INV is kept at 2.5V

by the local feedback of the error amplifier, a network connected between pins INV and

COMP that introduces a long time constant. Then the current through R2 remains equal to

2.5/R2 but that through R1 becomes:

Equation 2

The difference current

∆I

R1 = I’R1 - I’R1 = ∆VO/R1 will flow through the compensation network

and enter the error amplifier (pin COMP). This current is monitored inside the IC and when it

reaches about 18 µA the output voltage of the multiplier is forced to decrease, thus reducing

the energy drawn from the mains. If the current exceeds 20 µA, the OVP is triggered

(Dynamic OVP), and the external power transistor is switched off until the current falls

approximately below 5 µA. However, if the overvoltage persists (e.g. in case the load is

completely disconnected), the error amplifier will eventually saturate low hence triggering an

internal comparator (Static OVP) that will keep the external power switch turned off until the

output voltage comes back close to the regulated value. The output overvoltage that is able

to trigger the OVP function is then:

Equation 3

∆V

O = R1 · 20 · 10

-6

2.5

--------

2.5

–

----------------------

2.5

–

∆

----------------------------------------

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