LM2623A Efficiency

Ioannis Mourtsiadis1

Hi,

I'm designing a setup-up converter based LM2623A. It should to converts 3.7V to 5V and minimum 1.4 A, will be very good if provide 1.8A. I have designed the PCB as also assembled it. However the efficiency is very low, and far away from the data sheet. It is around of 58%, when drawing 1A, at 5V output and supply of 4.0V .Drawing higher current is not possible, because the output voltage drop down to 4.2 V on the output. It is also not possible to decrease the supply voltage below 3.8V, because it also results low voltage on the output.

Attached below link to schematic.

https://app.box.com/s/ed3ph9ffkew6d6c82uew

Any advice about my design would be very welcome.

Thank you in Advance

Grazyna

over 10 years ago

0 K Steenis over 10 years ago

Prodigy 140 points

MBRS130 is maximum 1A average, Make this one bigger. Maybe it is enough, but better safe then sorry.

Efficiency seems to drop a lot on higher currents, see datasheet figure 3.

0 Ioannis Mourtsiadis1 over 10 years ago in reply to K Steenis

Prodigy 100 points

Hi,

I changed it toSTPS340U - 3A Diode

http://gr.mouser.com/ProductDetail/STMicroelectronics/STPS340U/?qs=sGAEpiMZZMtQ8nqTKtFS%2fD9SVzsgHTKGAobHdv4aSbg%3d

Same effect. Are you sure that this chip meet the data sheet requirements, and can provide up to 2A output. Matter is critical, as I can not proceed with product development until I solve this issue. According to datasheet it can provide up to 2A, but is not. Please advice what other can I do.

Thank you in advance

0 K Steenis over 10 years ago in reply to Ioannis Mourtsiadis1

Prodigy 140 points

The datasheet says up to 2A at low output voltage, but it doesn't tell what a low output voltage is. Probably it means at the minimum output voltage of 1.24V.

Did you experiment with the C3 capacitor? Here you can find a better explanation:
http://www.ti.com/lit/an/snva050d/snva050d.pdf

0 Ioannis Mourtsiadis1 over 10 years ago in reply to K Steenis

Prodigy 100 points

Hi,

I played with C3, no significant difference. Who should know what the TI datasheet means ? Hope the TI can answer what is the low voltage. It is critical to decide for me if I can stay with this IC or go to another one. Do you know where I can find the answer if the low voltage is 5V, 3V or even 1.24V.

Thank you in advance

0 Howard Chen-WEBENCH Design Center over 10 years ago in reply to K Steenis

TI__Expert 6365 points

For highest efficiency, the circuit need to operate the lowest possible frequency. I don't know what your target design meant to be but it's a trade off of high efficiency means large overall footprint size vs. less efficient design meant to have smaller overall footprint size. Start by reducing the Rt value and you should see an improvement of efficiency.

Removed R16, and adjust C7 by monitoring the switch node to get a continuous switching waveforrm at the typical operating condition.

Suggestion of key components: Set R18 = 400k ohm, L1 = 7.6uH (Use a lowest DCR inductor) and use the suggested diode below.

http://www.vishay.com/doc?94178

0 Ioannis Mourtsiadis1 over 10 years ago in reply to Howard Chen-WEBENCH Design Center

Prodigy 100 points

Hi,

Thank you so much. I will follow your suggestions and report then as I finish the tests.

Warmest Regards

0 Anston Lobo over 10 years ago in reply to Ioannis Mourtsiadis1

TI__Genius 16025 points

Hi Loannis,

According to my calculations, with your inductor selection of 4.7uH, you will need a part that has a maximum Switch current of at least 4 A. The LM2623A is limited to 2A as spec'd in the current limit under the datasheet EC table.

With the LM2623A you will get about 800mA max load current at a decent efficiency for your 3.7V to 5V conversion.

For a boost Load current is NOT switch current and load current cannot be correctly spec'd as a general figure since it varies with boost voltage and frequency of operation.

I recommend using Webench to figure out your schematic and circuit needs available on the front page of TI.com.

Also, read this article http://www.ti.com/lit/an/slva372c/slva372c.pdf to learn more about the design equations you need.

I've attached a working design document here based on another part: 0753.5344.[E2E]Boost converter - Loannis.pdf

Thanks,

Anston

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LM2623A Efficiency