PM6686 datasheet(31/50 Pages) STMICROELECTRONICS

31 / 50 page

PM6686

Device description

Doc ID 15281 Rev 4

31/50

Figure 50.

Internal supply diagram

8.1.7

Current sensing and current limit

The PM6686 implements a positive valley current limit to protect the application from an

overcurrent fault. Each section has an independent current limit setting. A new switching

cycle can't start until the inductor current is under the positive current limit threshold. Note

that the peak current flowing in the inductor can reach a value greater than the current limit

threshold by an amount equal to the inductor ripple current.

Figure 51.

Current waveforms in current limit conditions

The inductor current is sensed during the off time TOFF by measuring the voltage drop

across the low-side MOSFET using the RDS(on) as a lossless sensing element (PHASE to

PGND voltage). The voltage drop is compared to the threshold set with ILIM pin. If ILIM is

connected to a voltage higher than VCC-1V the limit is 100mV. A current of 5 µA is sourced

from the pin ILIM; if a resistor is connected between ILIM and ground the current limit is

given by the voltage at the ILIM pin. The device sets the PHASE voltage threshold at 1/10 of

the ILIM voltage.

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