Application information

L6564

22/34

Doc ID 16202 Rev 1

Figure 35.

In this way a change of the line voltage will cause an inversely proportional change of the

half sine amplitude at the output of the multiplier (if the line voltage doubles the amplitude of

the multiplier output will be halved and vice versa) so that the current reference is adapted to

the new operating conditions with (ideally) no need for invoking the slow dynamics of the

error amplifier. Additionally, the loop gain will be constant throughout the input voltage

range, which improves significantly dynamic behavior at low line and simplifies loop design.

Actually, deriving a voltage proportional to the RMS line voltage implies a form of integration,

which has its own time constant. If it is too small the voltage generated will be affected by a

considerable amount of ripple at twice the mains frequency that will cause distortion of the

current reference (resulting in high THD and poor PF); if it is too large there will be a

considerable delay in setting the right amount of feedforward, resulting in excessive

overshoot and undershoot of the pre-regulator's output voltage in response to large line

voltage changes. Clearly a trade-off was required.

The L6564 realizes a NEW voltage feed forward that, with a technique that makes use of

just two external parts, strongly minimizes this time constant trade-off issue whichever

RFF, both connected from the pin VFF (#5) to ground, complete an internal peak-holding

circuit that provides a DC voltage equal to the peak of the rectified sine wave applied on pin

through the low impedance of the internal diode; in case of line voltage drop, an internal

“mains drop” detector enables a low impedance switch which suddenly discharges CFF

avoiding long settling time before reaching the new voltage level. Consequently, an

acceptably low steady-state ripple and low current distortion can be achieved without any

considerable undershoot or overshoot on the pre-regulator's output, like in systems with no

feedforward compensation.

peak amplitude that, with good approximation, is given by:

FF

FF

L

MULTpk

FF

C

R

f

4

1

V

2

V

+

=

Δ