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LM5170-Q1: External gate driver

Jochen Rohm
Prodigy 60 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170,

Hello,

i'm planning to use 2 LM5170 in a 4Phase configuration. Total power 3KW 12V/48V (62,5A per Phase).

My plan is to use the following MOSFET (two in parallel) IAUT300N10S5N015. Due to the massiv Cin and total gate charge i'm concerned that the internal driver of the LM5170 wont be able to support sufficient rise/fall times.Sadly i can't find the total power the gate drivers of the LM5170 are able to supply in the datasheet. Only the peak current of 4A/5A is given.

So my Question is the following:

# What is the maximum power delivery of the high and low side gate driver ?

# Can i connect an external gate driver ic to the lm5170, by using pin LO1 as signal input for external gate drive low side?

For the high side input i want to connect pin SW1/SW2 permanently to Ground, pin HB21/HB2 permanently to +10V (referenced to GND) and using pin HO1/HO2 as the input for the external gate drive high side. Is this possible?

Kind regards,

Jochen Rohm

  • 0
    TI__Guru* 78190 points
    Hi Jochen,

    Thank you for your interests in the LM5170-Q1. The gate driver strength is defined by the peak current it can deliver. The peak current determines the how fast the switching transient can be in order to minimize the switching losses. The switching transient can be estimated by the MOSFET total gate charge Qg: T_turn-on = Qg / Ipk_on; T_turn-off = Qg / Ipk_off

    We usually want to limit the on and off transient time to below 100ns, preferably at ~30 to 50ns, to minimize the switching losses.

    Now let me answer your questions:
    1. It is the matter of the MOSFET selection: Total Qg needs to be considered. Your MOSFETs much be large enough, mostly parallel MOSFETs for high power, to reduce the conduction losses. While, a lower Rds(ON) leads to higher Qg. Therefore, you need to make trade-offs in selecting the FETs, to balancing the switching and conduction losses such that the total losses are minimal.

    Based on my limited experience, 4phases for 3kW seems overstretching for the LM5170 internal drivers. I am more comfortable to recommend 6 phases.

    Another problem with 4phases 3kW is the availability of power inductors. Not many vendors carry inductors to support such high power.

    2. Yes, you can, for both the high side and low side external driver. You just need to make sure the dead time between the high side and low side is controllable with your external driver.

    Hope this clarifies, and good luck in your design.

    Best Regards,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 60 points
    Thank you very much for the fast and usefull response. Its exactly what i thought. So i have to use external drivers with integrated deadtime. The Inductor is no problem, since i already designed my own one. Again, thank you very much!