LM5010A-Q1: countermeasures to prevent through-current and to slow Fsw

Takao Yamamoto

Part Number: LM5010A-Q1
Other Parts Discussed in Thread: LM5010, , DRV8305

Dear all,

【QUESTION】

Q1. Adding chip beads is not recommended at data sheet. Are there something recommended countermeasures to through-current? In general, shorter Trr diode is recommended for free-wheeling diode. Therefore we use schottky diode with Trr = 7ns.

Q2. Could you teach about Gate drive UVLO detection delay time? Adding limiting resistor decrease Bst voltage under 4V when SW = ON. To set appropriate resistor value, we want the delay time information.

【Back ground】

We get trouble like following.

We use this device DC24V→DC12V and found EMI noise at our board from this device.

We found out the cause and found that through-current was flowing at free-wheeling diode connected SW pin.

For countermeasures, we added Snubber circuit, but could not decrease through-current.

We think that this is because of high transient current by fast Fsw and high input voltage.

>Following are our countermeasures to decrease through-current

1. Adding chip beads in series to diode to decrease transient current

2. Adding limiting resistor on Bst capacitance to slow Fsw

After these countermeasures, we found that LM5010 is stable and lower noise level.

Best regards,

over 8 years ago

0 Takao Yamamoto over 8 years ago

Genius 4135 points

Dear all,

Could anyone answer me?

Best regards,

0 JohnTucker over 8 years ago in reply to Takao Yamamoto

TI__Guru**** 191445 points

Let me see if I can find someone that supports LM5010A-Q1.

0 Takao Yamamoto over 8 years ago in reply to JohnTucker

Genius 4135 points

Hi John-san,

Thank you for your support.
Could you or answerer reply when you could answer this?

Best regards,

0 JohnTucker over 8 years ago in reply to Takao Yamamoto

TI__Guru**** 191445 points

I have forwarded your request. I cannot say when they may respond. I will ask again today.

0 Timothy Hegarty over 8 years ago in reply to JohnTucker

TI__Guru 73072 points

Yamamoto-san,

Please send the converter schematic and confirm what you mean by "through-current."

The following is a link to an EMI app note I wrote that discusses frequency components of EMI that arise from switching:

http://www.ti.com/product/LM5141-q1/technicaldocuments#doctype1

Here are some possibilities:

Add an RC snubber from SW to GND, e.g. 2 Ohms and 220pF.
resistor in series with the boot cap to slow the turn-on transition, e.g. 10 Ohms
Bring the GND plane closer to the power traces by reducing the PCB intra-layer spacing (this is described further in the app note above).
Ensure the dotted terminal of the inductor is tied to SW (such that the winding end that’s tied to SW is short and nearest the GND plane). Also, keep SW plane copper area as small as possible.

Regards,

Tim

0 Takao Yamamoto over 8 years ago in reply to Timothy Hegarty

Genius 4135 points

Hi Tim-san,

Thank you for your answer and recommendation.

I asked my customer to share schematic.

Sorry for my wrong English, and "through-current" means "shoot-through current". Is this clear?

We could decrease EMI level, but not shoot-through current.

Therefore, could you teach us how to take measures against shoot-through current additionally?

Best regards,

0 Takao Yamamoto over 8 years ago in reply to Timothy Hegarty

Genius 4135 points

Hi Tim-san,

I would appreciate if you inform some updates.

Best regards,

0 Sourav.Sen over 8 years ago in reply to Takao Yamamoto

TI__Intellectual 2320 points

Hello Yamamoto-san,

I am trying to fill in for Tim right now since he is Out of Office at present.

Can you please elaborate about the 'through current' that you have mentioned here. Aplogies about asking the same question again.

Regards,

Sourav

0 Takao Yamamoto over 8 years ago in reply to Sourav.Sen

Genius 4135 points

Hello Sourav-san,

Sorry for late response.

For example, 'trough current' is written as 'current shoot through' at p.15 of DRV8305 data sheet.

As data sheet says,

'short through' is the phenomenon that Hi-side and Low-side FET become ON together at switch transition time.

And 'shoot through current' is this flowing current.

Could you make sense? If you have question point, please let me know.

Best regards,

0 JohnTucker over 8 years ago in reply to Takao Yamamoto

TI__Guru**** 191445 points

Did this ever get resolved?

0 Timothy Hegarty over 8 years ago in reply to Takao Yamamoto

TI__Guru 73072 points

Yamamoto-san,

Since the LM5010A-Q1 is a non-synchronous converter, there is no shoot-through current as defined conventionally using high-side and low-side MOSFETs in a synchronous buck converter. Also, if the freewheeling diode is a Schottky, the reverse recovery current is minimal. One key point is to minimize the loop area of the input cap, IC and freewheeling diode - this is to minimize the "hot" loop parasitic inductance, which leads to SW node voltage ringing and increased emissions. See Figure 4 of this app note for more detail: www.ti.com/.../slyt682.pdf

Regards,

Tim

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LM5010A-Q1: countermeasures to prevent through-current and to slow Fsw