AN458 APPLICATION NOTE

2/19

Figure 2. Waveforms

The L4963 control loop is shown in the block diagram of fig.1 and the blocks that take part to the regulation loop,

are determined by the system operating conditions.

For a given value of the inductor L (that will be further calculated), if we examine the current across the inductor

inductor can have one of the shapes shown in fig. 2.

In both the waveforms, the current in the coil goes to zero during the Toff period of the power stage, but in the

case shown in fig. 2A there is a "Dead Time" during which, both the Power stage and the Free-Wheeling diode

a new cycle at the next set pulse coming from the clock.

In this way the system operates in fixed frequency mode, set by the External resistor RT connected between

pin. 17 and ground.

In fixed frequency mode, the current in the inductor reaches the peak value, determined by the load current,

following the law:

(1)

When the output voltage reaches its nominal value, the E/A output resets the power stage and the discharge

period starts.

When the power transistor turns off, the inductor will try to mantain the forward current constant, and the voltage

at pin.2 will fly negative until the diode D is brought into conduction. The current in the inductor L will now con-

tinue to circulate in the same direction as before, decreasing linearly from the peak value to zero following the

law:

(2)

It follows, then, a third period toff" (dead time) during which there is no current neither across the power transis-

tor, nor in the diode D, nor in the inductor L (the inductor runs ’DRY’). This period ceases when the next set

pulse from the clock circuit enables again the power stage repeating the cycle.

I

L

V

in

V

cesat

–

v

out

–

L

----------------------------------------------- t

on

⋅

=

I

L-

I

peak

V

out

V

F

+

L

------------------------- t

off

⋅

–

=