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L5994 - L5994A

Protections

Each converter is fully protected against fault conditions. A monitoring system checks for overvoltages of the

output, quickly disabling the interested converter in case such an event occurs. This condition is latched and to

allow the device to start again either the supply voltages have to be removed or the relative RUNx pin has to be

driven low.

Also the undervoltage conditions are detected: a light undervoltage (90% of the programmed value) only causes

the relative PWROKx to be driven low while an hard undervolatge (70% of the programmed value) causes in-

terruption of the operation of both converters. This is a protection against short circuits.

PWROKx signals (at pin 10 and 23) reveals the anomaly of the relative section (output voltage not within the

±10% of the programmed voltage) with a low output level. If the chip overheats (above 135°C typ.) the device

stops operating as long as the temperature falls below a safe value (105°C typ.). The overtemperature condition

is signalled by a low level on both PWROKx as well.

A current limitation comparator prevents from excessive current in case of overload. It intervenes as the voltage

VRSENSE exceeds 50mV, turning off the high-side switch before the error summing does. By the way, this also

gives the designer the ability to program the maximum operating current by selecting an appropriate sense re-

sistor. This pulse-by-pulse limitation gives a quasi-constant current characteristic.

Linear Driver

The Linear driver is capable of sinking up to 60mA from an external PNP transistor through the pin VDRLIN

considering the typical application circuit shown in fig. 4. The internal comparator is supplied by the same pin

VDRLIN which accepts voltages included in the range from 4.5V to 20V. If the application works with input volt-

ages that allows the regulation, the supply for the regulator can be obtained directly from the input source (VIN).

If such is not the case and is not available an additional input voltage, the most convenient way to get the supply

is to use an auxiliary winding on one of the two sections inductor with a catch diode, DS, and a filter capacitor,

CS, as shown in fig. 5. This winding delivers energy to pin VDRLIN during the recirculation period of each switch-

ing cycle with a voltage determined by the turns ratio n and little dependent on the input voltage.

Figure 4. Linear regulator supply with auxiliary winding

In case the section with the auxiliary winding is working at full load and the linear regulator is lightly loaded, the

voltage at pin VDRLIN can exceed the expected value. In fact, DS and CS act as a peak-holding circuit and

VDRLIN is influenced by the voltage spikes at switching transients. An internal clamp limits the voltage on the

VDRLIN pin at a maximum value of 16V, but, in case of intervention, the chip power dissipation will rise.The

linear driver is always active as long as PREG5 and VREF are present on the chip (see the relevant section); it

Vcc

VDRLIN

VFBLIN

2.5V

L5994

Vout