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I am trying to design a LED driver based the LM3409HV IC. I am using the reference design from TI as a starting point. The schematic in the datasheet has a diode (D2) and a resistor (R5) at the gate of the FET that switches current to the LED.

schematic from the datasheet

In the BOM, D2 is listed as "no load" and R5 is listed as having a 0 Ohm value. I have never seen this and can't understand what it means.

I have also seen other designs based on the same IC and the gate is connected straight to the IC, without any diode.

Can anyone explain to me why these components are included in the schematic, and if I can safely leave them out in my design ?

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    \$\begingroup\$ synertronic.co.za/post/2013/05/07/… does this help ? \$\endgroup\$ – efox29 Oct 17 '18 at 20:24
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    \$\begingroup\$ Same as R1 and R2 as it says "slow down the rising edge of the FETs slightly to prevent the gate from ringing" \$\endgroup\$ – Misunderstood Oct 17 '18 at 20:39
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    \$\begingroup\$ In the actual physical board the resistor is populated (though I don't have a multimeter handy to measure it), while the diode is left out. \$\endgroup\$ – feralgeometry Oct 17 '18 at 20:44
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    \$\begingroup\$ @UgoRiboni: It is typical to put in some extra components when you're doing a board, in case you need them, and then to leave them out or replace them with zero-ohm resistors (which is the equivalent to a wire or a jumper). The designer didn't know if they'd need something fancier than a direct connection, so they did some defensive design. \$\endgroup\$ – TimWescott Oct 17 '18 at 21:28
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    \$\begingroup\$ @ Ugo Riboni .D2 polarity is the oppositte of what you normally see .It is normal to turn on slower than turn off .The way D2 is you turn off slow and turn on fast .This will give more losses and more EMC issues .I think that it is a mistake that has not been spotted despite an accepted answer . \$\endgroup\$ – Autistic Oct 19 '18 at 10:54
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It is normal to drive the gate of a MOSFET through a resistor (normally under 100R). It is also seen a diode exactly as connected in the schematic.

The reason is that the gate is a capacitive load to the driver. This means that is has to charge when turned on, and discharged when turned off. If there is no resistor, the driver has to be capable of providing the peak current to charge the capacitor. On the other hand, to turn off the MOSFET you want it to discharge as fast as possible (for a fast turn off), and not through a gate resistor.

Obviously if you short circuit the resistor, there is no need for the diode. I'd say that in this case it's a reserve in the PCB to allow for MOSFETS with higher capacitance in the gate.

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That is not the schematic from the datasheet. The datasheet you linked is the user guide for the LM3409HVEVAL/NOPB-ND eval board.

D2 and R5 are not in any of the datasheet schematics.

You can safely leave them out.

The schematic you show is from the LM3409HV Eval board which has a circuit to demonstrate external parallel FET shunt for high frequency PWM dimming.

I believe D2 and R5 may have been related to the external PWM dimming circuit.

The LM3409 Eval board, PN: LM3409EVAL/NOPB-ND does not have the external PWM nor R5 or D2 footprints.

R5, if used, is the same as R1 and R2 as it says used to "slow down the rising edge of the FETs slightly to prevent the gate from ringing".

The zener foot print is on the PCB in the rare case you need to clamp voltage spikes.

My guess is that when TI designed the HV eval PCB they used a different MOSFET than what is now in the user guide BOM. This MOSFET change eliminated the need for D2 and R5 and the fix was to change R5 to a 0 Ω resistor. This is why these two components are not mentioned in the user guide.

With modern MOSFETs the switching speeds increase every year. The severity of the turn-off snap recovery is a function of the MOSFET switching speed. A MOSFET turn-on is what caused the diode to turn off in the first place. So, a simple solution is to slow the MOSFET down. Yes, why use fast MOSFETs. Well, we just want to slow down the MOSFET turn-on:

enter image description here

enter image description here

Source: Synertronic Designs thanks to @efox29

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