Pin connection
Table 2.
L6599A
Pin description (continued)
Type
Pin N#
Function
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).
4
RFmin
Burst mode operation threshold. The pin senses some voltage related to
the feedback control, which is compared to an internal reference (1.24 V).
If the voltage on the pin is lower than the reference, the IC enters an idle
state and its quiescent current is reduced. The chip restarts switching as
the voltage exceeds the reference by 50 mV. Soft-start is not invoked. This
function realizes burst mode operation when the load falls below a level
that can be programmed by properly choosing the resistor connecting the
optocoupler to pin RFmin (see block diagram). Tie the pin to RFmin if burst
mode is not used.
5
STBY
Current sense input. The pin senses the primary current though a sense
resistor or a capacitive divider for lossless sensing. This input is not
intended for a cycle-by-cycle control; therefore the voltage signal must be
filtered to get average current information. As the voltage exceeds a 0.8 V
threshold (with 50 mV hysteresis), the soft-start capacitor connected to pin
1 is internally discharged: the frequency increases, so limiting the power
throughput. Under output short-circuit, this normally results in a nearly
constant peak primary current. This condition is allowed for a maximum
time set at pin 2. If the current keeps on building up despite this frequency
increase, a second comparator referenced at 1.5 V latches the device off
and brings its consumption almost to a “before startup” level. The
information is latched and it is necessary to recycle the supply voltage of
the IC to enable it to restart: the latch is removed as the voltage on the Vcc
pin goes below the UVLO threshold. Tie the pin to GND if the function is
not used.
6
ISEN
Line sensing input. The pin is to be connected to the high-voltage input bus
with a resistor divider to perform either AC or DC (in systems with PFC)
brownout protection. A voltage below 1.24 V shuts down (not latched) the
IC, lowers its consumption and discharges the soft-start capacitor. IC
operation is re-enabled (soft-started) as the voltage exceeds 1.24 V. The
comparator is provided with current hysteresis: an internal 13 µA current
generator is ON as long as the voltage applied at the pin is below 1.24 V
and is OFF if this value is exceeded. Bypass the pin with a capacitor to
GND to reduce noise pick-up. The voltage on the pin is top-limited by an
internal Zener. Activating the Zener causes the IC to shut down (not
latched). Bias the pin between 1.24 and 6 V if the function is not used.
7
LINE
Latched device shutdown. Internally, the pin connects a comparator that,
when the voltage on the pin exceeds 1.85 V, shuts the IC down and brings
its consumption almost to a “before startup” level. The information is
latched and it is necessary to recycle the supply voltage of the IC to enable
it to restart: the latch is removed as the voltage on the VCC pin goes below
the UVLO threshold. Tie the pin to GND if the function is not used.
8
DIS
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Doc ID 15308 Rev 7
STMICROELECTRONICS [ ST ]
