L5994 - L5994A

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works in order to obtain a voltage on the VFBLIN pin of about 2.5V. In this way, the minimum regulated voltage

is of 2.5V, obtained connecting directly the VFBLIN pin to the output, while the maximum is of about the supply

voltage minus the bipolar PNP VceSAT.

For a correct operation of the regulator, the voltage at pin VDRLIN must not be too low. The flyback connection

of the two windings ensures a well regulated voltage, provided if there is good magnetic coupling. The coupled

inductors configuration, however, is not able to sustain the auxiliary voltage if the main output is lightly loaded:

the secondary voltage drops and the system goes out of regulation.

The additional winding may be implemented with L5994 if the relative section is loaded enough.

To overcome this problem, in L5994A, when the VDRLIN voltage falls below a certain threshold (13.7V ±5%)

because of too light a load on the section 2, the relevant synchronous rectifier is turned on for 1.5

µs max. during

the interval in which the inductor current is zero ("one-shot" feature, see fig. 6). In this way, the inductor current

reverses and draws from the output capacitor energy which is forward transferred to the auxiliary output.

Since that the linear driver is supplied from the VDRLIN pin, if the linear regulator is not necessary for the application,

leave floating this pin implies that the linear driver is not supplied and so no power is wasted (L5994 only).

The linear regulator is active, if at the least one of the two runx signal is asserted

Figure 5. "One Shot" pulse to substain VDRLIN voltage

+5V Linear Regulator and +2.5V Reference Voltage Generator

The 5V low drop-out regulator powers directly the MOSFET drivers and it is externally available through pin

PREG5. A low pass filter is connected between PREG5 pin and SREG5 pin from who all the internal circuitry is

powered. The introduction of this R-C network is useful to minimize noise effects.

The typical external use of this generator is to charge the bootstrap capacitors used to produce the gate-drive

voltage for the high-side MOSFET's of both PWM converters.

At start-up and when the 5V section is not operating, this regulator is powered by the chip input voltage. To re-

duce power consumption, the linear regulator is turned off and the PREG5 pin is internally connected to the 5V

PWM regulator output via V5SW pin, when the 5V PWM regulator is active and its output voltage is above the

switchover threshold, 4.5V. This happens when V5SW pin is connected to the section 1 output regulating 5V.

In any case, if V5SW is above 4.5V, the internal regulator is turned off and PREG5 is powered through this pin.

t

t

t

H1GATE

L1GATE

IL

1.5

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V13IN

13.7V