Design Calculations and Mode Transitions for Light Load

Samm Flanagan

Prodigy 100 points

Part Number: LMR66420
Other Parts Discussed in Thread: LMR33620, LMR36006, TPS61022, LM43603

Tool/software:

Hi, We are using LMR66420 for a design with the following requirements:

Vin = 24VDC,nom (19-29VDC)
Vout = 5VDC
Iout = ~70mA,nom (100mA,max)

We do not have stringent requirements on Vout ripple or transient respond speed.

My understanding from various TI documentation is that it will operate in PFM 100% of the time at the above loads.

The datasheet has limited information regarding the calculations required to select suitable components (Cin,Cout,L) when the part is in PFM.

I have tried using WEBENCH, but it is very limited in what information is available for this part as it does not enable the simulation part of WEBENCH.

I have been using the following to attempt to understand the expected operation of this device, could you please advise the suitability of these resources for the LMR66420 and if they are not applicable, please provide alternatives:

SNVC234 - Mode Transitions Calculator LMR336x0 LMR360xx
SNVA857 - Understanding Mode Transitions for LMR33620/30 and LMR36006/15
SSZTAL7 - Input and Output Capacitor Considerations in a Synchronous Buck Converter
SLVAFC3 - Mechanism of TPS61022 PFM and Load Threshold between PFM and PWM for TPS61xxx Converters
LM43603 Datasheet - Utilizing the component selection calculations

I am particularly interested in the following:

Component selection based on the design inputs when in PFM
Calculating the mode transition point out of PFM, based on component selection.
Vout ripple when in PFM

If there is a SPICE model for this converter, that would be ideal.

We are not considering using any alternative parts at this stage, unless the LMR66420 is expected to be unreliable in this configuration.

6 months ago

0 Pramanik Patel 6 months ago

TI__Expert 4930 points

Hi,
Following are the answers to the questions:

1) For component selection, inductor plays an important role. Below is the range in which you need to select inductor

Lower limit comes from the condition when the inductor current may hit the current limit and upper limit comes from the condition of DCM.

2) When you choose

Converter enters into CCM Mode

3) I need some time for this. Will get back to you latest by 27/03/2025 1:00PM.

I will also try to get the Spice model for this device.

Thanks,

Pramanik Patel

0 Samm Flanagan 6 months ago in reply to Pramanik Patel

Prodigy 100 points

Hi Pramanik Patel,

Thank you for the quick response.

I will review your answers to 1 & 2 and let you know if I have any further queries on these points.

I look forward to your update regarding my other queries.

Thanks!

0 Samm Flanagan 6 months ago in reply to Samm Flanagan

Prodigy 100 points

Hi Pramanik Patel,

I have had a go at calculating the Lmin & Lmax but am getting odd results, hopefully you can point out where I am going wrong:

0 Pramanik Patel 6 months ago in reply to Samm Flanagan

TI__Expert 4930 points

Hi,

I am very sorry; I made a mistake in the derivation. Following is the corrected one

Thanks,

Pramanik Patel

0 Pramanik Patel 6 months ago in reply to Pramanik Patel

TI__Expert 4930 points

Hi,

For the ripple voltage in output please refer the below application note:

Understanding Output Voltage Ripple in DCM Operation of D-CAP Buck Converters

Thanks,

Pramanik Patel

0 Samm Flanagan 6 months ago in reply to Pramanik Patel

Prodigy 100 points

Is it appropriate to use Ipeak = 2A or should we use Ipeakmin (0.5A,typ) from the datasheet?
How do we calculate what the expected switching frequency will be for our case? I see we are assuming the minimum in the above calculations.
What is D₁? It seems to be an order of magnitude less than D?

0 Pramanik Patel 6 months ago in reply to Samm Flanagan

TI__Expert 4930 points

Hi,
1) Ideally, we should select Ipeak = Ipeak max=3.9A in order to leverage the short circuit current feature.

2) D1 is the time when HS FET is ON.

During PFM we cannot predict the frequency, so best way is to design controller for max current and lowest possible frequency, so that you have short circuit protection feature available. Also, it will work for 100mA. So, my suggestion is to design for CCM and use it in PFM mode. You can also use LMR3650x 3-V to 65-V, 100-mA and 150-mA Wide VIN Synchronous Buck Converter Optimized for Size and Light Load Efficiency datasheet (Rev. A) and design for 150mA max.

Thanks,

Pramanik Patel

0 Samm Flanagan 6 months ago in reply to Pramanik Patel

Prodigy 100 points

If we should be using 3.9A, why have you used 2A in your above calculations?
I am still a bit confused about D₁, How have you calculated D₁ to be 0.0208 above?

0 Pramanik Patel 6 months ago in reply to Samm Flanagan

TI__Expert 4930 points

Hi,

1) Increasing current will lead to higher rating inductor, but choosing a lower current inductor will prevent you from using the short circuit protection feature.

And so, I would like to look at LMR3650x 3-V to 65-V, 100-mA and 150-mA Wide VIN Synchronous Buck Converter Optimized for Size and Light Load Efficiency datasheet (Rev. A) this will relax your inductor rating.

0 Samm Flanagan 6 months ago in reply to Pramanik Patel

Prodigy 100 points

Hi,

Are you recommending we change to LMR3650x or just read that datasheet?
1. I note in this datasheet it states "when selecting the ripple current for applications with much smaller maximum load than the maximum available from the device, use the maximum device current"
2. So to ensure we have short circuit protection, we should use the maximum high side switch current, correct?
Sorry, I think you misunderstand me. I understand that D₁ is the on time, but you haven't explained how you calculated it to be 0.0208=2.08%.
1. If we use CCM duty cycle calculation:
2. Using DCM:
  - = 0.2139 = 21.39%
3. Can you please show me your workings for calculating D=0.028?