Datasheet
DocID15308 Rev 8 5/37
L6599A Pin connection
37
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).
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