AN827 Datasheet, PDF (11/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

Where:
V_va-out = Error amplifier output voltage range
V_lff = V_ref = 5.1V if not used for load feed-forward
V_rms = Voltage at pin 7
I_IAC = Input current at pin 4
AN827 APPLICATION NOTE
To optimize the multiplier biasing for each application, the relationships between Imult and other input signals
are reported in the Designing A High Power Factor Switching Preregulator With The L4981 Continuous Mode
Application Note [1], Figures 13a-13h.
Pin 9 ISENSE (Current amplifier inverting input)
This pin is the current amplifier inverting input. It is externally connected to the network described at CA-OUT
(pin 5). Note that R14=R16=2.7k have the same value because of the high impedance feedback network. The
sense resistors, R30-R32, have a combined resistance of 0.033 ohms. The low value is chosen to minimize the
power losses since the total Inductor current flows through this resistor. The value must be large enough to pro-
vide a good signal to noise ratio signal to the current amplifier.
Pin 10 SGND (Signal ground)
This pin should be connected close to the reference voltage filter capacitor (C7). Refer to Figure 3, 500 Watt
Demoboard printed circuit board layout.
Pin 11 VREF (Voltage reference)
An external capacitor filter of 1µF, C7, should be connected from pin 11 (Vref) to ground. This reference voltage
of 5.1V is externally available and can deliver up to 10mA for external circuit needs such as the fast start-up
power supply circuit as described in Pin 19.
Pin 12 SS (Soft start)
This feature avoids current overload through the power mosfet during the ramp-up of the output boosted volt-
age. An internal switch discharges the capacitor if an output overvoltage (OVP) or a VCC undervoltage (UVLO)
is detected. The voltage at the soft-start pin acts on the output of the error amplifier and the soft start time is
calculated as follows:
t_ss = C_ss ^V-----^v--^a-_I--_s-^–-_s---^o---^u---^t = 1µF ₁---⁵-₀--^.-₀-¹---_µ-^V---_A-- = 51 ms
Where:
Css = C8 = 1µF
V_va-out = 5.1V is the typical error amplifier voltage swing
I_ss is the internal soft start current generator
11/20