LM3478 Adjustable UVLO Circuit

Suresh Samynathan

Other Parts Discussed in Thread: LM3478, LMV431, LM3481, LM5022

Hi,

Is thre any option available to implement the adjustable UVLO using external circuitry in LM3478. Basically I want to implement UVLO around 6V-7V.

over 13 years ago

0 Giuseppe Pinto over 13 years ago

TI__Expert 3950 points

Hello Suresh,

you could try to implement the following circuit:

Without the zener diode the LM3478 starts switching at Vmin = 3V

With the zener diode the LM3478 starts switching at Vmin = 3V + Vzener hence you could use a 3.3V zener for your application.

A more general circuit is the following:

It uses the LMV431, please take a look at the datasheet :

http://www.ti.com/lit/ds/symlink/lmv431.pdf

Hope that helps.

Regards,

Giuseppe

0 Giuseppe Pinto over 13 years ago

TI__Expert 3950 points

Anyway I really suggest you to use the LM3481 instead of the LM3478. The reason is that the LM3481 is much more reliable and it already includes a UVLO circuit, that means you just need to select two resistors.

0 Suresh Samynathan over 13 years ago in reply to Giuseppe Pinto

Intellectual 360 points

Hi,

Vin is conencted to decap after the 10 ohm resistor? I could not see full circuit.

0 Suresh Samynathan over 13 years ago in reply to Giuseppe Pinto

Intellectual 360 points

You have mentioned LM3481 is much more reliable than LM3478. Is thre any issues with LM3478.

0 Giuseppe Pinto over 13 years ago in reply to Suresh Samynathan

TI__Expert 3950 points

Hello,

yes Vin is connected to the bypass cap after the 10 ohm resistor.

About the LM3478:

1. the part can reach 100% duty cycle, that means once it hits 100% it just shorts the input and it can't recover. That event can occurs pretty often especially if there is a low current limit at the input power supply.

2. The internal LDO regulates the internal voltage when Vin is higher than 7V, otherwise Vin is just connected directly to the internal circuitry. Unfortunately, on the edge of this transition (around 7V) the part generates a lot of jittering on the switching point although the output voltage is still regulated.

As I wrote before I suggest you to use the LM3481.

Regards,

Giuseppe

0 Suresh Samynathan over 13 years ago in reply to Giuseppe Pinto

Intellectual 360 points

Hi,

is there any difference in efficiency between LM3478 and LM3481.

When I simulate in Webench, i could found around 1% efficiency less in LM3481. Our requirements are VIN=8.1V-35.2V, VOUT=12V, Iout=0.12A (max). what will be the efficiency at 12V input, SEPIC Mode.

0 Giuseppe Pinto over 13 years ago in reply to Suresh Samynathan

TI__Expert 3950 points

the LM3478 as well as the LM3481 is a low side controller. That means it does not really affect the efficiency.

The efficiency is really defined by power stage component selection, switching frequency.

If webench generated a slightly different solution is probably due to components selection and/or different switching frequency.

Please use WEBENCH to get an idea of the expected efficiency.

0 Thida Khaing over 12 years ago in reply to Giuseppe Pinto

Prodigy 20 points

Hi ,

I am using LM3481 as step up regulator .Maximum input is 15V and minimum is 6.3V(below minimum Vin, the regulator shouldn't output anything).

Output voltage is 12V, Iout is greater than or equal to 200mA.

To do so,Can I used UVLO settings to get above feature? If then, is it the right way to set UVLO threshold to 6.3V(that defines under voltage) ?

and What would be the two programming resistors values?

Kindly advice.Thanks.

Thida

0 Anston Lobo over 12 years ago in reply to Thida Khaing

TI__Genius 16025 points

Hi Thida,

For your application, please consider the attached design from WeBench based on LM5022 which is quite similar to LM3481.

0676.LM5022 - 12V SEPIC - Precision Enable design.pdf

Thanks,

Anston

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LM3478 Adjustable UVLO Circuit