A5974AD datasheet(14/49 Pages) STMICROELECTRONICS

14 / 49 page

Functional description

A5974AD

14/49

Doc ID 018762 Rev 1

Figure 6.

Synchronization example

5.4

Current protection

The A5974AD features two types of current limit protection; pulse-by-pulse and frequency

foldback.

The schematic of the current limitation circuitry for the pulse-by-pulse protection is shown in

Figure 7. The output power PDMOS transistor is split into two parallel PDMOS transistors.

The smallest one includes a resistor in series, RSENSE. The current is sensed through

RSENSE and, if it reaches the threshold, the mirror becomes unbalanced and the PDMOS is

switched off until the next falling edge of the internal clock pulse. Due to this reduction of the

ON time, the output voltage decreases. As the minimum switch-on time necessary to sense

the current, in order to avoid a false overcurrent signal, is too short to obtain a sufficiently

low duty cycle at 500 kHz (see Section 8.4), the output current in strong overcurrent or

short-circuit conditions may not be properly limited. For this reason, the switching frequency

is also reduced, therefore keeping the inductor current under its maximum threshold. The

frequency shifter (Figure 5) functions based on the feedback voltage. As the feedback

voltage decreases (due to the reduced duty cycle), the switching frequency decreases also.

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