FS6407 datasheet(47/111 Pages) NXP

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NXP Semiconductors

FS6407/FS6408

6.7.1.4

Overcurrent detection and current limitation

6.7.1.4.1

Overcurrent protection:

In order to ensure the integrity of the high-side MOSFET, an overcurrent detection is implemented. The regulator is switched OFF by the

main state machine when the overcurrent detection threshold IPRE_OC is reached three consecutive times. The overcurrent detection is

blanked when the pass transistor is switched ON during TPRE_OC to avoid parasitic switch OFF of the high-side gate driver.

The VPRE output voltage decrease causes an undervoltage condition on one of the cascaded regulators (VCORE, VCCA, VAUX) and

bring the device in Fail-safe state. The overcurrent protects the regulator in case of SW_PRE pin shorted to GND. The overcurrent works

in Buck mode only.

6.7.1.4.2

Current limitation:

A current limitation is also implemented to avoid uncontrolled power dissipation inside the device (duty cycle control) and limits the current

below IPRE_LIM. The current limitation is blanked when the pass transistor is switched ON during TPRE_BLK_LIM to allow short-circuit

detection on SW_PRE pin.

When IPRE_LIM threshold is reached during Buck mode, the high-side integrated MOSFET is switched OFF. When IPRE_LIM threshold is

reached during Boost mode, the external low-side MOSFET is switched OFF. In both cases, the MOSFET is not switched ON again before

the next rising edge of the switching clock.

The current limitation induces a duty cycle reduction and leads to the VPRE output voltage to fall down gradually. This may cause an

undervoltage condition on one of the cascaded regulators (VCORE, VCCA, VAUX) and bring the device into Fail-safe state. The current

limitation does not switch OFF the regulator. The current limitation protects the regulator when VPRE pin is shorted to GND.

Figure 31. Overcurrent and current limitation scheme

SW_PRE

IPRE_OC

IPRE_LIM

IPRE_SW

TPRE_BLK_ILIM

TPRE_OC

VPRE

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