A5974D datasheet(13/46 Pages) STMICROELECTRONICS

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A5974D

Functional description

5.6

PWM comparator and power stage

This block compares the oscillator sawtooth and the error amplifier output signals to

generate the PWM signal for the driving stage.

The power stage is a highly critical block, as it functions to guarantee a correct turn-ON and

turn-OFF of the PDMOS. The turn-ON of the power element, or more accurately, the rise

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 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

Vgsmax

GATE

STOP

DRIVE

DRAIN

OFF

PDMOS

VOUT

DRAIN

VCC

ILOAD

ESR

AM00009v1

IOFF

ION

ON/OFF

CONTROL

CLAMP

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