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LM5146-Q1: How to connect VCC, PGOOD. And is it okay add Gate/G-S resistor in external FET?

Hyojin Kim
Prodigy 10 points
Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5145-Q1

Tool/software:

Dear TI

I designed the circuit 24V to 12V using in LM5146-Q1. 

1.VCC Pin connection

In eval board schematic, VCC Pin connect to out(12V). 
But in the datasheet, VCC is internal regulator generating at 7.5V. 

Does it have any advantage in this connection?? 
In datasheets, some effecient is better, but I think it's okay to remain NC to VCC. 

If VCC doesn't connect to anywhere, is it possible to work wrong or happen serious problem?? 


2. PGOOD

In eval board schematic, it connect to VCC with pull-up. 
If I don't need the PGOOD Function, is it possible to remain NC in PGOOD Pin?? 

3. added the stable resistor in external MOSFET 

In eval board, there is not any resistor around external mosfet. 
But about 10ohm Gate resistor helps to avoid rush current or control switching frequency. 
And about 10Kohm resistor between Gate and source can disspate electric on gate and prevent boot loss. 

I want to make this on the circuit, is it okay to use like below? 

  • +1
    TI__Genius 11425 points

    Hi Hyojin.

    Hyojin Kim said:

    1.VCC Pin connection

    VCC pin - as you rightly pointed out is the 7.5V regulator output. The LDO generates a stable output from the Vin input. However, if there is an external bias supply that is connected to this pin (lower than the abs max value of this pin) - the power consumption within the IC reduces since it has to step down from the external bias supply and not high voltage Vin. 

    See 8.3.3 High-Voltage Bias Supply Regulator (VCC) in the datasheet for more information. 

    If you don't want to use the external Bias option - it is not recommended to leave the pin as NC. Connect a ceramic decoupling capacitor between 1 µF and 5 µF from VCC to AGND for stability

    Hyojin Kim said:

    2. PGOOD

    Yes, you can leave it open or tie it to GND if not used.

    Hyojin Kim said:

    3. added the stable resistor in external MOSFET 

    Yes you can add the gate resistor in your design as well as the Gate Source Resistor in your design.

    However, please be aware that these are lossy elements in your circuit and will add power consumption and make the efficiency of the design worse. I would recommend that you use only if necessary. I have seen multiple designs work perfectly fine (including the EVM) without using these resistors. 

    Thanks,

    Best Regards,

    Taru

  • +1 in reply to Taru Kapoor
    TI__Guru 73682 points

    Just to add what Taru mentioned, putting a resistor on the low-side FET gate is a bad idea (it causes Cdv/dt shoot-though). The EVM has a 2.2Ω resistor in series with the boot cap - this is sufficient to control high-side FET switching if needed.

    PS: If Vin is only 24V, you can use the LM5145-Q1 (rated to 75V input) instead of the LM5146- Q1 (100V). 100V FETs may not be needed here (60V or 80V may be sufficient).

    --

    Tim