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L6590

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 L6590D 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

disables 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. An internal comparator continuously monitors the Vcc voltage and stops the operation of the

IC if the voltage exceeds 16.5 V. This condition is latched and maintained until the Vcc voltage falls below the

UVLO threshold. The converter will then operate intermittently.

Figure 36. 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).

Vcc

DRAIN

+

-

to OVP

latch

GND

to

MOSFET

OVP