RT2910A

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is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS2910A-00

August

2017

14

0.6V

0.2V

OCP/OVP

Trigger Signal

1.25V

Figure 1. OCP/OVP Fault Sequence

If the fault condition persist, fault sequence as Figure 2.

HV switch is operation, but fault condition is still exist,

by 3.5uA until 3ms time out. Because POK (ENHV) pull

again.

0.6V

0.2V

OCP/OVP

Trigger Signal

TMR

GATE TO OUT

FDLY

POK

1.25V

3ms TMR blank time

1.45V

3.5uA discharge

Decay Down

25uA charge CTMR

Figure 2. OCP/OVP Fault Condition Persist Sequence

MOSEFT Selection

The N-Channel MOSFET load switch is the critical

component for the protection circuit. Choosing an

appropriate device is not difficult but there are many

important requirements. The most important are :





maximum current rating



maximum drain-source voltage



maximum gate-source voltage



power dissipation and safe operating area (SOA)



gate threshold (for lower VIN applications)

For most of the time the MOSFET will be fully on. In that

state, the voltage loss and power dissipation are a

that doesn’t drop more voltage than is acceptable

considering the minimum value the intended input

voltage and the voltage requirements of the load, and

one that can handle the required continuous current.

Avoid logic-level

MOSFETs with their low VGS

maximum ratings, or add a GATE-VOUT clamp to avoid

damaging. The RT2910A GATE drive voltage may be

as high as 14V so standard-threshold MOSFETs with

20V VGS ratings are recommended.

When the MOSFET is turned off (whether in shutdown

or in OVP or OCP) the full input voltage appears across

the MOSFET. Choose a MOSFET with a maximum

drain-source voltage exceeding your maximum input

surge voltage.

During an over-voltage (OV) event the MOSFET will

linear regulate the output voltage delivered to the load.

According to the timing determined by the capacitor

connected at the TMR pin, the circuit will turn the load

on and off periodically until the over-voltage ends. While

linear-regulating, the MOSFET will dissipate power and

heat up. Since TMR charges at twice the rate that it

discharges, the MOSFET will linear regulate with a duty

cycle of about 12% during a long continuous OV event.

If the OV event is shorter than the TMR charge timing

then examine the MOS ET’s safe operating area (SOA)

for

the

drain

current, to

determine if the over-voltage event will cause MOSFET

MOS ET’s SOA limits.

If the OV event lasts more than one TMR cycle then the

MOSFET will turn on and off, dissipating power each

time it is on and linear regulating and cooling down

when it is off. In this case, use one of the longer-timed

areas of the SOA graph but adjust

the

drain current

value by the 12% duty cycle of the MOSFET on periods

determine if the MOSFET will work. For thermal

management, the MOSFET dissipation during long

over-voltage events is :

where DC is the duty cycle of linear regulation, typically

about 12%.