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LM74610-Q1: Why Gate is not 100% ON

Jawad EL SAYED
Prodigy 20 points
Part Number: LM74610-Q1
Other Parts Discussed in Thread: LM74700-Q1

Hello,

As per LM74610-Q1 datasheet, we read that the IC doesn't drive 100% the gate ON for the N-MOSFET transistor even in normal conditions. In fact, we have 98% of the time ON, 2% OFF.

This has 3 major drawbacks:

1. 2% of the time the power dissipation will have the diode high dissipation factor (that we are looking to avoid when doing reverse polarity protection circuits) instead of small RDSON related dissipation

2. the VOUT will show some ripple due to switching between MOSFET and DIODE modes. The ripple is in fact a voltage drop (around 0.7V to 1V which is considered big if we intend to supply a 5V input)

3. MOSFET keeps switching all the time ! MTBF issues? switching noise?

My question related to this issue are:

Q1. how to avoid such VOUT drop or minimize it to lower its impact on the next stage DC/DC converters?

Q2. Is there any benefit of the 98%-2% switching mechanism that I am not aware off? If not, can we find another IC reference that do the same task as the LM74610-Q1 while keeping 100% gate ON when VIN is OKay?

  • 0
    TI__Guru 57737 points

    Hi Jawad,

    LM74610-Q1 is a Smart Diode Controller which does not have a GND connection. 

    The charge pump circuit  in this device needs to have some voltage difference as input and for this the body diode drop  (when the FET is turned OFF) of the FET is utilized. This forward voltage drop (Vf ) across the body diode of the MOSFET is used to charge up the charge pump capacitor Vcap. 

    You can refer to section 7.3 Feature Description of the datasheet to understand more about this.  All the ideal diode controllers which do not have GND reference follow the same technique to produce a gate voltage from Charge pump which is greater than input voltage. 

    In case you want an Ideal diode controller which provides continuous gate voltage, please consider using devices like LM5050, LM74700-Q1 , LM7480x-Q1