LM5176: diode requirement

Hi Fred,

thank you for using the E2E forum.

Please check this section from the datasheet on the usage of the blocking diode:

Best regards,

 Stefan

hi Stefan,

just to confirm, so this is not necessary in my case, right?

Hi Feng,

i can not answer this with the available information

For low VIN operation, ensure that the VCC voltage is sufficient to fully enhance the MOSFETs. Use an external bias supply if VIN dips below the voltage required to sustain the VCC voltage.

-> Vin is high enough for get Vcc regulated. Is this then enough for your FETs to properly pass the Miller plateau.

For these conditions, use a series blocking diode between the input supply and the VIN pin (Figure 7-1). This prevents VCC from back-feeding into VIN through the body diode of the VCC regulator

-> you are not using an external Bias supply - right?

If both answers to this questions are yes then you do not need it.

Best regards,

 Stefan

Hi Stefan,

thanks reply

I'm not sure what else available information you need to make judgement ?

Vin may be 9V,

Vout=12V but Bias pin is connect to Vout,

Does this mean BIAS will keep VCC from back-feeding even Vin is only 9V?

and which mean I don't need the diode, is that right?

thanks

Regards,

Fred

Hi Feng,

I would need this two answers:

-> IsVcc high enough for your FETs to properly pass the Miller plateau?

-> you are not using an external Bias supply - right?

Most important is the second one - if you are not using an external Voltage to supply the BIAS pin you do not need the diode

Best regards,

 Stefan

hi Stefan,

1. what confuse me is , I've said the bias pin is connect to Vout, 

does vout seen as external supply to you by definition?

since I'm not sure if we have mutual understanding on this

2. the blocking diode is to prevent back-feeding.

Even we use external BIAS supply which exceed VCC by one Vf, 

it should not happen back-feeding since VCC is regulated by Bias instead of low Vin,

 so blocking diode is not needed.

above is my understanding. please correct me

Hi Fred,

1. yes, VOUT is an external connection.

2. if Vcc is source out of VBias and Vin drops below Vcc then the diode (Body Diode) on the internal FET will conduct and source Vin out of Vcc.

This will be prevented by the blocking diode.

Best regards,

 Stefan

Hi Stefan,

Really appreciate your patient reply, I got it now.

would you mind I change the subject that what is the spec of this blocking diode?

If the Input current may be 12A for my customer, then below diode average forward current is only 2A, which is not suitable ,right?

Hi Feng,

we are still discussing the diode on Vin - right.

So we have 4mA from the logic and the we need to add the driver current for the low side FET : I(FET) = 2 {FET} * Q(Gate,FET) * U{VCC} * Fsw * 2  {both edges}

See {} as comments in the formula.

So 12A is the input current for the power stage but not for VIN. For Vin it should be more in the range of ~ 50 - 300 mA (mainly defined by the FET).

~ Stefan