LP8552 minimum PWM pulse on-time

tateo yamashiro

Guru 10890 points

Other Parts Discussed in Thread: LP8552

Hi, My customer has question about LP8552. Could you help us?

Q: How much is maximum value of minimum PWM pulse on-time at the over temperature range? *typical value is 1usec.

Best Regards,

tateo

over 9 years ago

0 Harini Sankabattula over 9 years ago

TI__Intellectual 1780 points

Hi Tateo,

As you pointed out, DS put a typical 1uS as the min PWM pulse on-time. I'll check with our Characterization team to see if they have over temp data on this parameter.

However, why do you care about minimum PWM Pulse On-time?

Thank you,
-Harini

0 tateo yamashiro over 9 years ago in reply to Harini Sankabattula

Guru 10890 points

I'm sorry for the long delay in my reply.
My customer considers to use PWM Dimming frequency is up to 20kH and minimum On-Time is 2usec.
So they has concern about variation of minimum PWM pulse on-time.

Best Regards,
tateo

0 Harini Sankabattula over 9 years ago in reply to tateo yamashiro

TI__Intellectual 1780 points

Hi Tateo,

As long as the customer does not go below 1us PWM ON time, design operates correctly.

Best,
-Harini

0 tateo yamashiro over 9 years ago in reply to Harini Sankabattula

Guru 10890 points

Thank you for your support.
My customer has one more question.

- My customer wants to use L=4.7uH or 3.3uH under the following conditions.

   Vin=5~21V
   Vout=36V
   Iout=20mA/ch
   fsw=1.25MHz

Is there any concerns about this usage?
They has concern about the stability.

Best Regards,
tateo

0 Harini Sankabattula over 9 years ago in reply to tateo yamashiro

TI__Intellectual 1780 points

Hi Tateo,

The inductor value that was chosen here seems less than what is recommended in the datasheet. Please go over the DS page #s 36 & 37.

What is the Isat and DCR of the L they want to use. Can you share your L datasheet?

Thankyou,
-Harini

0 tateo yamashiro over 9 years ago in reply to Harini Sankabattula

Guru 10890 points

Thank you for your reply.
My customer considers to use DFE252010F-4R7M.
Could you tell us the concern if inductor value is less than recommended value?

Best Regards,
tateo

0 Harini Sankabattula over 9 years ago in reply to tateo yamashiro

TI__Intellectual 1780 points

Thank you Tateo.

The Isat of this part meets the calculations of >1.23A. However, I've not used such a low inductor in this application. The Boost ripple would be on the high side (0.74A with 21Vin, 36Vout, 4.7uH & 1.25MHz).

What output Cap do they plan to use? I'd say ~10uF.

It is better to configure an eval board with these specifications and run stability analysis to be sure.

Best,
-Harini

0 tateo yamashiro over 9 years ago in reply to Harini Sankabattula

Guru 10890 points

Thank you for your reply.
They can use ~10uF. They should use lower than 1mm height following parts.
So they will use 4.7uF in several parallel.

If they want to run stability analysis, could you provide EVM or simulate stability?

Best Regards,
tateo

0 Harini Sankabattula over 9 years ago in reply to tateo yamashiro

TI__Intellectual 1780 points

Hi Tateo,

Yeah two 4.7uF in parallel can be used instead of one 10uF.

Do they have the unit with them? It looks like there is no EVM on the TI website to order. I'll look around in our lab to see if we can find any here. However, shipping an EVM from our product group is a lengthy process.

Best,
-Harini

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LP8552 minimum PWM pulse on-time