L6566ATR datasheet(40/51 Pages) STMICROELECTRONICS

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Application information

L6566A

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the base terminal are several ten kW) but this could distort the signal on the MULT pin of the

PFC chip and adversely affect the operation of the pre-regulator. CF needs to be quite a big

capacitor (in the uF) to have small residual ripple superimposed on the dc level; as a rule-of-

thumb, use a time constant (RL + RH)·CF at least 4-5 times the maximum line cycle period,

then fine tune if needed, considering also transient conditions such as mains missing

cycles.

If temperature effects are critical, the NPN Q can be replaced by a PNP-NPN pair arranged

as shown in Figure 26 on page 39 on the right-hand side; other sensing techniques can be

adopted anyway.

The voltage on the pin is clamped upwards at about 3.15 V; then, if the function is not used

the pin has to be connected to Vcc through a resistor (220 to 680 k

Ω).

5.13

Slope compensation

The pin MODE/SC (12), when not connected to VREF, provides a voltage ramp during

MOSFET's ON-time synchronous to that of the internal oscillator sawtooth, with 0.8 mA

minimum current capability. This ramp is intended for implementing additive slope

compensation on current sense. This is needed to avoid the sub-harmonic oscillation that

arises in all peak-current-mode-controlled converters working at fixed frequency in

continuous conduction mode with a duty cycle close to or exceeding 50%.

Figure 27. Slope compensation waveforms

The compensation will be realized by connecting a programming resistor between this pin

and the current sense input (pin 7, CS). The CS pin has to be connected to the sense

resistor with another resistor to make a summing node on the pin. Since no ramp is

delivered during MOSFET OFF-time (see Figure 27), no external component other than the

programming resistor is needed to ensure a clean operation at light loads.

Note:

The addition of the slope compensation ramp will reduce the available dynamics of the

current signal; thereby, the value of the sense resistor must be determined taking this into

account. Note also that the burst-mode threshold (in terms of power) will be slightly

changed.

If slope compensation is not required with FF operation, the pin shall be left floating.

Internal

oscillator

(pin 4)

MODE/SC

(pin 12)

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