L6564H STMicroelectronics, L6564H Datasheet - Page 20

• Manufacturer Part Number: L6564H
• Description: Power Factor Correction ICs High Voltage Startup Trans-Mode PFC 700V
• Manufacturer: STMicroelectronics

Specifications of L6564H

• Rohs: yes

Application information
20/35
In this way a change of the line voltage causes 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 is 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 is constant throughout the input voltage range, which
significantly improves 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 is affected by a
considerable amount of ripple at twice the mains frequency which causes distortion of the
current reference (resulting in high HD and poor PF); if it is too large there is a considerable
delay in setting the right amount of feedforward, resulting in excessive overshoot and
undershoot of the pre-regulator output voltage in response to large line voltage changes.
Clearly a trade-off was required.
The L6564H realizes a new voltage feedforward that, with a technique that makes use of just
two external parts, strongly minimizes this time constant trade-off issue whichever voltage
change occurs on the mains, both surges and drops. A capacitor C
both connected from the VFF pin (#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 the MULT
pin (#3). In this way, in the case of sudden line voltage rise, C
the low impedance of the internal diode; in the case of line voltage drop, an internal “mains
drop” detector enables a low impedance switch which suddenly discharges C
long settling time before reaching the new voltage level. The discharge of C
its voltage equals the voltage on the MULT pin or if the voltage on the VFF pin falls below
0.88 V, to prevent the “brownout protection” function from being improperly activated (see
Section 6.6: Power management/housekeeping functions
As a result of the VFF pin functionality, an acceptably low steady-state ripple and low current
distortion can be achieved with a limited undershoot or overshoot on the pre-regulator
output.
The twice-mains-frequency (2
peak amplitude that, with good approximation, is given by:
where f
ripple, related to the amplitude of its 2
Figure 33
amount of maximum desired 3rd harmonic distortion. Note that there is a minimum value for
the time constant R
occur. In fact, the twice-mains frequency ripple across C
must be lower than the minimum line drop detection threshold (DVVFF_min = 40 mV).
Therefore: must be lower than the minimum line drop detection threshold (∆V
mV).
L
is the line frequency. The amount of 3
shows a diagram that helps in choosing the time constant R
FF
×C
FF
below which improper activation of the VFF fast discharge may
Doc ID 022960 Rev 2
•
f
L
) ripple appearing across C
∆
D
V
•
3
FF
%
f
L
=
component, is:
=
1
2
+
π
2
f 4
V
rd
f
100
L
MULTpk
R
L
R
harmonic distortion introduced by this
FF
FF
C
C
FF
FF
FF
).
under steady-state conditions
FF
FF
is triangular with a peak-to-
is rapidly charged through
FF
FF
and a resistor R
· C
FF
FF
FF_min
FF
is stopped as
based on the
avoiding a
L6564H
= 40
FF
,