l6590a STMicroelectronics, l6590a Datasheet - Page 15

• Manufacturer Part Number: l6590a
• Description: Fully Integrated Power Supply
• Manufacturer: STMicroelectronics

Another problem is the spurious restarts that are likely to occur during converter power down if the input voltage
decays slowly (e.g. with a large input bulk capacitor) and that cause the output voltage not to decay to zero
monothonically.
Converter shutdown can be accomplished with the L6590A by means of an internal comparator that can be used
to sense the voltage across the input bulk capacitor. This comparator is internally referenced to 2.5V and dis-
ables the PWM if the voltage applied at its non-inverting input, externally available, is below the reference. PWM
operation is re-enabled as the voltage at the pin is more than 2.5V.
The brownout comparator is provided with current hysteresis instead of a more usual voltage hysteresis: an in-
ternal 50 µA current generator is ON as long as the voltage applied at the non-inverting input exceeds 2.5V and
is OFF if the voltage is below 2.5V. This approach provides an additional degree of freedom: it is possible to set
the ON threshold and the OFF threshold separately by properly choosing the resistors of the external divider,
which is not possible with voltage hysteresis.
Overvoltage Protection
The IC incorporates an Overvoltage Protection (OVP) that can be particularly useful to protect the converter and
the load against voltage feedback loop failures. This kind of failure causes the output voltage to rise with no
control and easily leads to the destruction of the load and of the converter itself if not properly handled.
If such an event occurs, the voltage generated by the auxiliary winding that supplies the IC will fly up tracking
the output voltage. This will activate an internal clamp circuit and, as the current sunk by this clamp exceeds
about 10 mA, the operation of the IC will be stopped. This condition is latched and maintained until the Vcc volt-
age falls below the UVLO threshold. The converter will then operate intermittently.
Figure 30. OVP internal schematic
Overcurrent Protection
The device uses pulse-by-pulse current limiting for Overcurrent Protection (OCP), in order to prevent overstress
of the internal MOSFET: its current during the ON-time is monitored and, if it exceeds a determined value, the
conduction is terminated immediately. The MOSFET will be turned on again in the subsequent switching cycle.
As previously mentioned, the internal powerMOSFET has a SenseFET structure: the source of a few cells are
connected together and kept separate from the other source connections so as to realize a 1:100 current divider.
The "sense" portion is connected to a ground referenced, sense resistor having a low thermal coefficient. The
OCP comparator senses the voltage drop across the sense resistor and resets the PWM latch if the drop ex-
ceeds a threshold, thus turning off the MOSFET. In this way the overcurrent threshold is set at about 0.65 A
(typical value).
GND
CLAMP
Vcc
+
L6590A
-
OVP
to OVP
latch
DRAIN
MOSFET
to
L6590A
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