TPS61088 - Is external current limiting method required for protection?

Sergey Feingold

Other Parts Discussed in Thread: TPS61088

Hello,

I'm having a bit of a debate with one of our engineers here. We are attempting to drive a resistive load (0.5 Ohm) up to a maximum power of 18W. Our current boost circuit using the TPS61088 is designed for a maximum of 18W (9V @ 2A). We need to control the power going through the load from 0-18W.

Right now we have a second (buck) converter (STM LED2001) attached to the output of the TPS61088, which regulates the power as needed. However the parts count and footprint for 2 converters is large. My colleague is suggesting we remove the buck converter and just use the TPS61088, with the output connected to the load through a simple MOSFET switch, and the power output is controlled by PWM of the MOSFET.

Here is my concern with this: while the MOSFET is turned on, the 0.5 Ohm load on the TPS61088 @ 9V will try to draw 18A - 162W! Obviously the TPS61088 can't provide this, but it seems to me we are effectively shorting the output every time the MOSFET turns on.

So my question is - is this a safe way to control the power going to a low-resistance load? Or do we need to make sure that we don't exceed the power rating of the TPS61088 with external circuitry?

Thank you for any help!

over 9 years ago

0 Helen chen over 9 years ago

TI__Mastermind 40080 points

Yes, you should make sure the load is not exceeding the TPS61088's maximum output capability. 18A-162W is really too big.

Since the output power of the TPS61088 is 18W, so you can choose a higher operation frequency like 1MHz to reduce the size of the indcutor and capacitors. a 1uH inducotr is fit for your applicaiton when fsw=1MHz. It is easy to found a small 1uH inductor. I can recommended one to you if you need it.

Similarly, you can choose a high frequency buck converter to reduce the size.

0 Sergey Feingold over 9 years ago in reply to Helen chen

Prodigy 220 points

Thank you for the reply! I will discuss it with my colleague. Our inductors are quite small already, but the final board size has to be very compact so even those small inductors are undesirable, but unavoidable. I am using this schematic from the TI WebBench tool: http://i.imgur.com/8HZ6wf3.png

I believe the operating frequency is 2.2MHz. The inductor we are using is about the smallest I could find: www.digikey.com/.../5050895

But if you know of a smaller one I'd love to know about it! The maximum height we can use is 3.0mm however.

Thank you again for the help.

- Sergey

0 Sergey Feingold over 9 years ago in reply to Sergey Feingold

Prodigy 220 points

One more thing Helen: do you have any suggestions for how to control the output efficiently, preferably without adding another converter? Looking forward to your response!

- Sergey

0 Helen chen over 9 years ago in reply to Sergey Feingold

TI__Mastermind 40080 points

Thanks for the update. 2.2MHz operation frequency is really too high, it is not good for the efficiency optimization. I suggest you use 1.4-1.5 MHZ operation frequency, in this condition, an 1uH inductor is enough, coilcraft 4mm*4mm inductor is fit for this appplication. Please check the attached datasheet. For the output cap, suggest the effective capacitance should be higher than 8.6uF@ 100mV output ripple. I noticed there's only 1pcs 10uF cap at the output side, what is the datasheet for this cap, did you check the effective capacitance? Or the output voltage ripple will be much bigger bigger.xal4020-102ME.pdf

0 maxxx over 8 years ago in reply to Helen chen

Prodigy 40 points

Hi Helen,

I'm using TPS61088 in a project (Fsw ~ 1.2 MHz), but seems this chip cannot deliver more than 8W,

Even with TPS61088 EVM Board, we couldn't receive more than 8W, any advise would be helpful

Thanks,

0 Helen chen over 8 years ago in reply to maxxx

TI__Mastermind 40080 points

what is the input voltage, output voltage in your applicaiton when the maximum output power is only 8W?

For the TPS61088EVM, when Vin=3.3V, Vo=9V, Io can be as high as 3A.

0 maxxx over 8 years ago in reply to Helen chen

Prodigy 40 points

Vin = 3.7 v

Vout = 9 v

Iout = less than 1A

0 Helen chen over 8 years ago in reply to maxxx

TI__Mastermind 40080 points

If you test with the EVM board, this should not be happended.

Did you make some change on the EVM board?

Did you test the input voltage on the EVM board (not the input voltage from the supply ), because the voltage drop across the input cable will big if the cable is thin and long.

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TPS61088 - Is external current limiting method required for protection?