Hello,
on a board I am using a TPS71750 (DSE type) in order to power: ADS1274 (75 mA max in high resolution mode), THS4524 (5.2 mA max), OPA2365 (10 mA max) and three accelerometers (10 mA max each). The total max current is about 120 mA. I have to add some little more current for a voltage divider which constitutes the reference voltage for the sensors, but that's less than 1 mA.
The problem is that TPS71750 does not seem to work as I would expect. With a 5.8 V input, the output is about 1.6 V. This appears to me as an overcurrent problem, but TPS71750 is stated to source up to 150 mA (and more, looking at the data sheet). Moreover, those listed above were maximum values, which I would only expect to find at extreme temperatures.
I double checked my schematics and found nothing should be wrong. What more can I check? Is there anything more I should be aware of?
And, should it really be an overcurrent problem, is there any 5 V LDO rated for 200 mA+ AND with the same DSE package (6-SON 1.5x1.5 mm)?
Thanks in advance,
Stefano Infante
Hi Stefano,
Can you attach a schematic of your application with the TPS717 so that we can look at it in detail?
Does the LDO start up to the correct output voltage when the load is removed, if so, then yes, there may be a larger load at the output than expected. Unfortunately, we do not have a pin-pin device with a higher current rating for the TPS717. We do have some fixed 5V LDO with different packages like the TPS7350 (500mA), TPS76750, and TPS76850 (1A) that are fixed and have higher current. But there would be more options with an adjustable version.
Regards,
Darwin
Hello Darwin,
here you can find the schematics concerning TPS717. Please note that the third part is to be intended x3 (as there are three identical parts for the three axes X, Y, Z), and that on the second part the jumper is not mounted while the 2 resistors marked as "do not mount" are actually mounted, in order to let TPS717 supply the 2.5 V reference for the three sensors.
From other tests, I found that the overall current drawn by the circuit at 6 V is 120 mA, but the circuit also has digital power supplies and other stuff. Moreover, if I try to shut down the ADC, the voltage on the TPS717 output goes down to 0.8 V...
Thanks for your help,
Stefano
Thanks for the schematic.
From your description it seems that it may well be the load that is causing this. Have you checked to see if the output starts up correctly with the load removed?
Your schematic seems okay. But here are some recommendations:
What is resistance of the fuse? Or actually, what is the voltage right at IN? Be sure you are not at dropout. What are the voltage ratings of your input and output capacitors? 1uF (effective capacitance) is recommended. You may need to increase this. How far are your capacitors from the pins of the IC. Typically you want them as close as possible to the IC's pins.
Hi Darwin,
I made another test: I disabled the 5V output on the board and connected a 5V source directly on the TPS717 output; this way, I can measure the effective current drawn by the 5V line, and it is about 35 mA, so I don't think I have a load problem. I removed the load (by simply not powering the ADC lines), but nothing seems to change; on the contrary, the voltage on the output goes down to 0.8V. I did not try to physically disconnect the ADC from TPS717, as it would be a little more difficult.
About the recommendations: the fuse does not seem to be the problem, as I can correctly see the 5.8 V line after the fuse (taken directly on TPS717); as TPS717 is stated to have a 0.3 V dropout, I am sure I am not in dropout. The input and output capacitors are AVX 0805YC105JAT (X7R, 1 uF, 16V), so I don't think they could be the problem. The capacitors are quite near the chip (I guess a few millimeters).
Now I also have another problem: the ADC does not seem to work, even with this external 5V input. Maybe I have to check it all again. But I don't know whether the two problems can be connected or not...
Thanks again,
The external components you chose are okay.
From the description above, if the output is dropping even when using an external supply, it seems like the ADC (or some other load) is causing the power supply to current limit and drop the output voltage. Be sure that no abs max conditions are being violated for any of the components. Possibly a short somewhere under some condition or maybe the board layout. Regards,
Hi， Stefano
I met the same problem. The symptom is very similar with yours. Output voltage is only about 0.8~1.0v when getting loads.
I finally found the TPS71750 is put in reverse direction on the PCB board. You know this chip is very tiny... . After the direction problem is fixed, it works.
I suggest you to check that.
Zou Yao
Darwin,
maybe I was not so clear in explaining what I did... with the external supply, the ADC does not work correctly, but the power supply is not current limited and stands correctly at 5 V; so I have now two different problems: TPS71750 is not providing the correct voltage (but this has nothing to do with overcurrent, as the external power supply only draws a few tens milliamps), AND the ADC is not working.
Now I have to solve both...
Hi Yao,
did you find the output voltage to be 1.3 V with no load? I will surely check the direction, but it apparently is ok. If you use the same package (DSE), can you please confirm the dot on the top of the chip is located on the pin #1, as usual? On the data sheet I did not find any information about this... it only says pin #1 is specified by a longer pin, but nothing is written about what can be found on its top...
Hi, Stefano
I also use DSE package. With no load, the chip outputs 5.2v with 6.0v input, with load, it drops down to 0.8v.
I cannot see the dot clearly, but only the "PD" mark on the chip. If yours is the same, the correct direction (from the top view) is as follows, the first pin is below the char 'P' :
6 5 4___ __ | PD |---------1 2 3OUT GND NR/FB
Hope it's useful to you.
Yao,
this is very useful to me, as it allows to know for certain how the chip is mounted on my board. Unfortunately, it is mounted in the correct direction (with pin #1 on the chip corresponding to pin #1 on the PCB), so I have to look somewhere else. By the way, in my board the output voltage is 1.36 V when the load is disconnected (ADC in power down) and it goes down to 0.8 V when I try to switch on the ADC output.
Thanks again.
In lab under the external components and operating conditions Vin=5.8V, and Iout=120mA, I have a 5V output. Possibly the ADC issue and LDO issue are related. It would be best to check the LDO in a separate circuit (with an external input power supply and resistive load) or somehow cut/disconnect the trace connecting the output of the LDO to all the loads to know for sure. If the IC is operating correctly, it must be the loads or the PCB layout.
I think you're right: it must be something in the ADC circuit causing both problems. I have an ADC evaluation board: I will use it to check the board ADC circuit, and see what's wrong. At this point, I have little doubt about the TPS71750, it will probably work correctly if I disconnect all loads.
After this check, I will write here again to get more advise from you :)
I solved the problem: it was my fault, on the board schematics I used the SC70 pinout for TPS71750, but on the PCB I used the WSON version, so the pins do not correspond to the PCB footprint.