L4984D STMicroelectronics, L4984D Datasheet - Page 16

• 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
5
5.1
16/35
Application information
Theory of operation
The L4984 implements conventional “peak” current mode control, where the on-time TON of
the external power switch is determined by the peak inductor current reaching the
programmed value. The off-time TOFF, instead, is determined by a special fixed-off-time
(FOT) modulator in such a way that the resulting switching period is constant as long as the
boost converter is operated in CCM (i.e. the current in the boost inductor remains greater
than zero in a switching cycle).
To understand how TOFF needs to be modulated to achieve a fixed switching frequency
independent of the instantaneous line voltage and the load, it is useful to consider the V·s
balance equation for the boost inductor under the assumption of CCM operation:
Equation 1
where Vpk is the peak line voltage, Vout the regulated output voltage and θ the
instantaneous phase angle of the line voltage. Solving for TON, we get:
Equation 2
then, the switching period T
Equation 3
In the end, if TOFF is changed proportionally to the instantaneous line voltage, i.e. if:
Equation 4
then T
f
voltage and the output load are such that the boost inductor operates in CCM.
SW
= 1/T
SW
SW
is equal to K
T
sw
) is fixed. This result is based on the sole assumption that the instantaneous line
=
T
ON
+
t
·V
T
T
out
OFF
ON
SW
and, since V
Vpk
=
Doc ID 023489 Rev 3
is:
T
⎛
⎜ ⎜
⎝
ON
sinθ
Vpk
T
=
Vout
OFF
⎛
⎜ ⎜
⎝
=
sin
Vpk
out
=
T
Vout
OFF
θ
K
is regulated by the voltage loop, also T
−
t
sinθ
Vpk
(
1
Vout
⎞
⎟ ⎟
⎠
T
−
sin
OFF
1
⎞
⎟ ⎟
⎠
−
θ
T
+
Vpk
OFF
T
OFF
sinθ
=
Vpk
)
Vout
sin
θ
T
OFF
SW
L4984
(and