TPS40170 does not start

Kent Sullwold

Other Parts Discussed in Thread: TPS40170

I have a TPS40170 design that I originated with TI's Webench app. Input voltage is 40V (+/- 5%) and the output voltage is 24V @ 2.5A. We have ~300uF of capacitance on the output (6 x 47uF @ 35V tantalum). With no DC load on the output, the output will often go into a hiccup mode (overcurrent ?). I observe 3 or 4 steps of output voltage perhaps 1, 3 & 5V ) and then the output collapses for a few seconds then tries again. If I isolate the regulator from all other circuitry (including the caps) the output always comes up.

Looking at the Webench generate documentation Ilim is 48.7K but when I calculate Ilim for 2.5A I get 1.58K.

I also have 2 other TPS40170 regulators on same board with same 40V input: 12V @ 2.5A and 5V @ 2.5A. They both seem to function correctly all the time.

Your assistance would be appreciated.

thanks

Kent S.

over 10 years ago

0 Anthony F over 10 years ago

TI__Mastermind 35115 points

Hi Kent,

Sorry for the delayed response. Please see my comments below.

How different is the layout between the different regulators? Layout is very important when it comes to nuissance hiccup tripping.
What values did you use to calculate 1.58kΩ?
Could you also share the WEBENCH design report for this design and if there is any difference your final schematic?

Thanks,
Anthony

0 Kent Sullwold over 10 years ago in reply to Anthony F

Prodigy 70 points

Anthony,

Sorry about the huge delay, burning issues on another project.

1) The layouts are essentially identical to each other and to the TPS40170 eval board layout

2) From the eqn on pg 16 of the spec Rilim = (Ioc X Rdson)/9.0uA. I am using the TI CSD18563 FET (5.7mOhm), Ioc = 2.5A.

How can I attached files to this thread?

thanks

Kent Sullwold

Note: This regulator starts up with virtually no DC load and ~ 300uF of capacitance in addition to what you see in the schematic screen shot.

Thanks

0 Kent Sullwold over 10 years ago in reply to Kent Sullwold

Prodigy 70 points

Anthony - attached is the top side layout of the 24V regulator on my PCB

0 Kent Sullwold over 10 years ago in reply to Kent Sullwold

Prodigy 70 points

A screen shot of the schematic

0 Kent Sullwold over 10 years ago in reply to Kent Sullwold

Prodigy 70 points

The bottom side of the PCB layout

0 Kent Sullwold over 10 years ago in reply to Kent Sullwold

Prodigy 70 points

The TI Webench generated design package for the 24V regulator, 40V input, 2.5A out

24V_Design_Pkg_ 40V In.pdf

0 Anthony F over 10 years ago in reply to Kent Sullwold

TI__Mastermind 35115 points

Hi Kent,

Thanks for sharing all the details. They layout looks good. It seems some of the GND copper didn't fill in, correct me if I'm wrong.

When calculating the current sense resistor, it is important to include:

The ripple current in the inductor
variation in the Rdson of the FET
Some margin for transients, including current need to charge the output capacitors during startup

Some details on this are shown in Equation 39 on the TPS40170 datasheet.

Have you tested with R(ILIM) increased? If not, can you check if this causes the issue to go away?

Anthony

Power management

Power management forum

TPS40170 does not start