L4984D STMicroelectronics, L4984D Datasheet - Page 18

• Manufacturer Part Number: L4984D
• Description: Power Factor Correction ICs Current Mode PFC CCM Line Modulated Bi
• Manufacturer: STMicroelectronics

Specifications of L4984D

• Rohs: yes

Application information
18/35
Along a line half-cycle, TOFF goes all the way from a minimum near the zero-crossing to a
maximum on the sinusoid peak. It is important to check that the off-time occurring on the
peak of the voltage sinusoid at minimum input voltage is greater then the minimum
programmable value:
Equation 9
This constraint limits the maximum programmable frequency at:
Equation 10
As the line RMS voltage is increased and/or the output load is decreased, the boost inductor
current tends to become discontinuous starting from the region around the zero-crossings.
As a result, the switching frequency is no longer constant and tends to increase. However,
the frequency rise is significantly lower as compared to that in a transition-mode (TM)
operated boost PFC stage, as illustrated in
fsw.max in the region where the inductor current is discontinuous.
Figure 27. Typical frequency change along a line half-cycle in a boost PFC operated
In this example the voltage ripple appearing across the output capacitor Cout has been
neglected. This ripple at twice the line frequency fL has peak amplitude ΔVout proportional
to the output current Iout:
Equation 11
As a consequence, fsw is not exactly constant but is modulated at 2fL, which spreads the
spectrum of the electrical noise injected back into the power line and facilitates the
1.6
1.4
1.2
0.8
0.6
0.4
1
0
in LM-FOT (left) and TM (right)
Vin = 264 Vac
0.52
Vin = 230 Vac
Line voltage phase angle (rad)
LM -FOT operated PFC
1.05
Vin = 115 Vac
Vin = 88 Vac
T
OFF
1.57
f
Doc ID 023489 Rev 3
sw
min
.
2.09
max
=
Δ
I
Vout
TIMER
=
2.62
C
690
T
3.14
=
K
Figure 25
Vpk
P
4
Vout
Vpk
π
Iout
f
L
min
9
8
7
6
5
4
3
2
1
0
0
Cout
min
. The switching frequency can exceed
[kHz]
0.52
>
Transition -mode operated PFC
1.45
Line voltage phase angle (rad)
Vin = 230 Vac
1.05
Vin = 115 Vac
Vin = 264 Vac
μ
s
1.57
2.09
Vin = 88 Vac
2.62
3.14
AM13243v1
L4984