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UC2909-EP: Question about the topology

Wayne Wang20
Expert 4380 points
Part Number: UC2909-EP

Hi team,

My customer is developing UC2909 circuit. And I have some question about below application circuit.

In the power part topology, I think it is a Buck structure. And the MOS should be a PMOS.

When UC2909 OUT pin=0, Point A=Vin*R2/(R1+R2). If Point A>Vin/2, Point B=PointA-0.7V. If Point A<Vin/2, Point B=Point A+0.7. So we need to set a proper R1 and R2 to make the voltage difference between 'Vin' and 'Point B' large enough to switch on the PMOS.

When UCC2909 OUT pin=OC, Point A=Vin, Point B=Vin-0.7V, So PMOS is switched OFF.

I am not sure if my analysis is correct. Could you please help double check on it? Thanks.

Best regards，

Wayne

  • 0
    TI__Guru**** 165766 points

    Hi Wayne,

    Yes, the topology is buck and the mosfet must be a PMOS since there is no bootstrap or charge pump to increase the gate voltage for an NMOS.

    The stacked bipolar transistors are a simple push pull gate driver.  Your analysis is correct but here are other things to consider:

    I(OUT)PEAK = 100mA <= VIN/(R1+R2)

    The size of the resistors gets IBASE for each bipolar and IBASE*Beta is the gate turn on and off voltage. 

    Regards,

    Jeff