Datasheet

Functional description B5973D

14/44 DocID14117 Rev 4

time of the current at turn ON, is a very critical parameter. At a first approach, it appears that

the faster the rise time, the lower the turn ON losses.

However, there is a limit introduced by the recovery time of the recirculation diode.

In fact, when the current of the power element is equal to the inductor current, the diode

turns OFF and the drain of the power is able to go high. But during its recovery time, the

diode can be considered a high value capacitor and this produces a very high peak current,

responsible for numerous problems:

 Spikes on the device supply voltage that cause oscillations (and thus noise) due to the

board parasites.

 Turn ON overcurrent leads to a decrease in the efficiency and system reliability.

 Major EMI problems.

 Shorter freewheeling diode life.

The fall time of the current during turn OFF is also critical, as it produces voltage spikes (due

to the parasites elements of the board) that increase the voltage drop across the PDMOS.

In order to minimize these problems, a new driving circuit topology has been used and the

block diagram is shown in Figure 8. The basic idea is to change the current levels used to

turn the power switch ON and OFF, based on the PDMOS and the gate clamp status.

This circuitry allows the power switch to be turned OFF and ON quickly and addresses the

freewheeling diode recovery time problem. The gate clamp is necessary to ensure that V

of the internal switch does not go higher than V

max. The ON/OFF Control block protects

against any cross conduction between the supply line and ground.

Figure 8. Driving circuitry

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