A5974AD datasheet(16/49 Pages) STMICROELECTRONICS

16 / 49 page

Functional description

A5974AD

16/49

Doc ID 018762 Rev 1

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 therefore noise) due to

the board parasites.

●

Turn-on overcurrent leads to a decrease in the efficiency and system reliability.

●

Major EMI problems.

●

Shorter free-wheeling diode life.

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

to the parasitic 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 (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

free-wheeling diode recovery time problem. The gate clamp is necessary to ensure that VGS

of the internal switch does not go higher than VGSmax. The on/off control block protects

against any cross conduction between the supply line and ground.

Figure 8.

Driving circuitry

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