DocID028175 Rev 3

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L6599AF

Pin connection

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3

Pin connection

Figure 2. Pin connection (top view)

Table 2. Pin description

Pin no.

Type

Function

1Css

Soft-start. This pin connects an external capacitor to GND and a resistor to RFmin (pin 4)

that set both the maximum oscillator frequency and the time constant for the frequency shift

that occurs as the chip starts up (soft-start). An internal switch discharges this capacitor

every time the chip turns off (Vcc < UVLO, LINE < 1.24 V or > 6 V, DIS > 1.85 V, ISEN > 1.5

V, DELAY > 2 V) to make sure it is soft-started next, and when the voltage on the current

sense pin (ISEN) exceeds 0.8 V, as long as it stays above 0.75 V.

2DELAY

Delayed shutdown upon overcurrent. A capacitor and a resistor are connected from this pin

to GND to set the maximum duration of an overcurrent condition before the IC stops

switching and the delay after which the IC restarts switching. Every time the voltage on the

ISEN pin exceeds 0.8 V, the capacitor is charged by an internal 150 µA current generator

and is slowly discharged by the external resistor. If the voltage on the pin reaches 2 V, the

soft-start capacitor is completely discharged so that the switching frequency is pushed to its

maximum value and the 150 µA is kept always on. As the voltage on the pin exceeds 3.5 V

the IC stops switching and the internal generator is turned off, so that the voltage on the pin

decays because of the external resistor. The IC is soft-restarted as the voltage drops below

0.3 V. In this way, under short-circuit conditions, the converter works intermittently with very

low input average power.

3CF

Timing capacitor. A capacitor connected from this pin to GND is charged and discharged by

internal current generators programmed by the external network connected to pin 4 (RFmin)

and determines the switching frequency of the converter.

4RFmin

Minimum oscillator frequency setting. This pin provides a precise 2 V reference and a

resistor connected from this pin to GND defines a current that is used to set the minimum

oscillator frequency. To close the feedback loop that regulates the converter output voltage

by modulating the oscillator frequency, the phototransistor of an optocoupler is connected to

this pin through a resistor. The value of this resistor sets the maximum operating frequency.

An R-C series connected from this pin to GND sets frequency shift at startup to prevent

excessive energy inrush (soft-start).