AN827 Datasheet, PDF (10/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
f_z = ^f-₄-^s---_π^w-- = -₂----_â‹---_π-----_â‹---_C--¹---₉-----_â‹---_R-----₁---₅-- therefore,
C9 = -_R----₁----₅--²--_â‹---_f--_S----_w-- = 692pF use C9 = 680pF
Pin 6 LFF (Load feed-forward input)
This pin allows the modification of the multiplier output current proportionally to the load in order to improve the
load transient response time. This function is not used in this circuit and the pin is connected to VREF.
Pin 7 VRMS (Voltage input)
This function is very useful for universal input mains applications to compensate the gain variation related to
the input voltage change. This pin is connected through an external network to the rectified line input. The best
control is achieved when the VRMS voltage level is in the range of 1.5 to 5.5V.
To avoid the rectified mains line ripple (2f), a two pole low-pass filter is realized with R3-R6 and C1-2. The lowest
pole is set near 3Hz and the highest pole near 13 Hz to reduce the gain to -80dB at 100 Hz.
V_rmspin7
=


----------------------^R-----³------------------------^
R3 + R4 + R5 + R6^
V_r
m
s
l
i
n
e
f_pole1 = ₍---_R-----₅----+-----_R--¹---₆----₎---_â‹---_C-----₂-- = 3.66Hz
f_pole2 = _R-----₄----¹-_â‹---_C-----₁-- = 12.6(3.66)H z
Where:
R3 = 33kΩ, R4 = 360kΩ, R5 = R6 = 620kΩ,
C1 = C2 = 220nF
At 88 Vrms, Vpin7 = 1.78 Vrms
At 264 Vrms, Vpin7 = 5.33 Vrms
Gain at 2f (100Hz) = -80dB
For single mains operation, this pin can be connected directly to Vref (pin 11) or to ground and the RC network
can be removed. If connected to ground, the Vrms multiplier input is clamped at 1.5V.
Pin 8 MULT-OUT (Output of the Multiplier)
This pin delivers the current Imult that is used to fix the reference voltage for the current amplifier. Pin 8 is con-
nected through R14 to the negative side of the sense resistor, R30-32, to sum the (I_L · R_s) and the (I_mult · R14)
signals, where I_L is the inductor current. The sum is the error voltage signal at the current amplifier non-inverting
input. The multiplier output current is determined by the equation given below:
I_mult = 0.37 ⋅ I_AC ⋅ ⁽---^V----^v---^a----^–---^o---^u---^t---–-----¹---^.-²----⁸---_V-^V----_r⁾-_m---^â‹--_s-⁽-₂-⁰----^.-⁸-----^â‹---^V----^l-^f--^f--–---¹---^.--²---⁸----^V----⁾- = I_IAC ⋅ ⁽---^V----^v---^a----^–---_V^o---^u-_r--^tm---–--_s--₂-¹---^.-²----⁸----^V----⁾-
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