L6566BH datasheet(39/51 Pages) STMICROELECTRONICS

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L6566BH

Application information

Doc ID 16610 Rev 2

39/51

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

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

Ω).

5.13

Slope compensation

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

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

minimum current capability. This ramp is intended for implementing additional 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 is realized by connecting a programming resistor between this pin and

the current sense input (pin 7, CS). The CS pin must 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 reduces 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) is slightly changed.

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

5.14

Summary of L6566BH power management functions

The device is provided with a number of power management functions: multiple operating

mode upon loading conditions and protection functions. To help the user familiarize

themselves with these functions, in the following tables, all themes are summarized with

their respective activation mechanism and the resulting status of the most important pins.

This can be useful not only for a correct use of the IC but also for diagnostic purposes:

especially at the prototyping/debugging stage, it is quite common to come across unwanted

activation of some function, and these tables can be used as a kind of quick troubleshooting

guide.

AM11503v1

Internal

oscillator

(pin 4)

MODE/SC

(pin 12)

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