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TPS61251: Using TPS61251 as boost converter to power MOTOR driver IC

Ahmet Inan
Intellectual 450 points
Part Number: TPS61251
Other Parts Discussed in Thread: TPS62082, , TPS61252

Hi TI Support team,

I have designed (PCB produced) a board that will be used to control some sensors (humidity, temperature etc). The board is also designed to drive two DC motors (3-6V, 250 RPM, total max 500mA current under load for two of them) and I used TB6612 as dual motor driver. The motor driver has built in back emf protection. 

My board has two input sources USB and  1s 2200mAh 18650 Li-ion battery. I am just using USB to program the MCU on the board. The battery voltage is converted to 3.3V through TPS62082 to power MCU and sensors. The battery voltage is converted to 5V through TPS61251 and 5V is only used to power motor driver IC. There is no problem regarding 3.3V. In my previous board, voltage input was 4xAA battery and I used MC33063 to get 5V and power Motor driver and other sensor and MCU part and the PCB worked well. 

I have problem regarding 5V. When I check the voltage on 5V out of TPS61251 it is ok. But when I connect it to motor driver IC (0 ohm jumper) and drive motors two or three times (for short time), there becomes short to ground on 5V part. When I replace the tps61251 IC with new one, it is again ok and motor works for some short time, but then short to ground again occurs.

1. My  first question is whether TPS61251 is suitable to power motor driver IC? And as I see there is stock problem regarding this component, could you please offer me, a new IC that can be used for my 5V part? I used TI web search for IC selection but not sure if I will see same problem on my board?

2. The component placement on my board for TPS61251 is very similiar to datasheet PCB layout part. My PCB is two layer, bottom gnd and top 3.3V. I attached the schematic and PCB placement. If the problem is regarding my PCB design could you please comment on it? 

Thank you for your help,

Ahmet

  • 0
    TI__Genius 9046 points

    Hi Ahmet,

    Thanks for reaching out and so detailed explanation on your questions and application. I did not see problems in your schematic or layout. Please kindly see below comments from me.

    1. I do not think there will be specific requirement regarding power supply of motor driver IC. Do you have any waveform of operation which you can share to us? Any waveform like output voltage of TPS61252 before attaching to motor driver, and when it is driving motor driver. I want to check what is the happening when the device the supplying the load. 

    2.Would you mind unpopulated the "damaged" IC and use multi-meter to check connectivity between SW-GND and SW-VOUT?

    2.Are you using TPS61251 or TPS61252? They are different part and has different operation mode in light load.

    -Wenhao

  • 0 in reply to Wenhao Wu
    Intellectual 450 points

    Hi Wenhao,

    I am sorry for my delayed response. I did not have access to my lab for past few days so I was not able to reply to you with the results.

    Thank you for your comments.

    1. Actually, I observed some "interesting" thing on my PCB. Before connecting the 5V rail to the load side through 0 ohm jumper I measured the voltage on both sides. Without any voltage connected to 5V load side, I measured 0.115V on load side. When connecting the 5V part to the load side I found that the 5V started to drop towards 0.5V in time. When disconnecting 3.3V from circuit (removing R20), there was not any problem either 5V connected to load or not connected. But when 3.3V was present (R20 connected) and I connect the 5V rail to my 5V load, the voltage was again dropping towards 0.5V. In my PCB, I made PCB top layer 3.3V (whole top layer) and 5V tracks was passing through this 3.3V polygon. I think the reason for the problem is top layer design (making whole plane 3.3V and passing 5V tracks through it). Somehow the plane affects the 5V tracks I guess. 

    I redesigned my top layer and removed whole 3.3V plane. Instead this time I used 3.3V tracks and 5V tracks and this time there is no power plane on top layer. I will produce new PCB and check if it will solve the problem. Do you think this solution may work? 

    2. Because my observation I mentioned in 1. the IC I removed was not damaged definetely (and as I threw it, I was not able to check the connectivity you mentioned. I am sorry for it)

    3. Actually, I have both TPS61251 and TPS61252 and due to their same footprint, pinouts and circuitry I was using them interchangeably. When I designed my board, there was stock in Digikey for TPS61251, but now it is out of stock. So I changed the schematic to TPS61252 due to similiar behaviour.

    I again would like to thank you for your help. 

    Ahmet

  • +1 in reply to Ahmet Inan
    TI__Genius 9046 points

    Hi Ahmet,

    Thank you so much for further testing and experiment.

    1.TPS62082 is a buck converter which output contains ripple at switching frequency but I do not think this would influence control too much as Buck converter's ripple noise is usually smaller than a boost converter. However, in a practical way it is not recommended to place TPS61252's FB close to the noise source as this would disturb the control.

    There are some practical way to further improve your layout:

    1).Remove dead copper island which can be a noise "receiver"

    2).Consider move the R8 closer to IC.

    3).Use solid GND polygon at bottom side behind IC. This is minimize parasitic parameters.

    4).Consider add a via near R8 to short GND connection.

    5).On top side, use GND pour instead of 3V3 pour is a good way to prevent noise;

    2.It is recommended to measure the SW-GND and SW-VOUT to confirm damage before throw it to trash bin;

    3.TPS61252 has larger quiescent current than TPS61251 and it is not optimized for large output capacitance. For your case, 22uF is ok to be used here. 

    -Wenhao

  • 0 in reply to Wenhao Wu
    TI__Genius 9046 points

    Hi Ahmet,

    Another point when using TPS61251 for motor driver IC is manually force motor reverse rotate. Reverse rotate could produce a back electromotive force and charge up Vout. When Vout is charged above TPS61251's abs max rating (7V), the device could break. You can test and monitor Vout when using motor driver IC to check this. Thanks!

    -Wenhao  

  • 0 in reply to Wenhao Wu
    Intellectual 450 points

    Hi Wenhao,

    Thank you for the point. Actually, I did not think about it. 

    The point you mentioned will most probably happen. I designed this board as a present for my child, and most probably he will manually rotate the wheels back and forth and as you mentioned this will create back emf and may damage the IC. Can I use schottky diode clamps on Vout rail to protect it similiar to ADC input protection? Or is there any alternative way to protect Vout?

    -Ahmet  

  • +1 in reply to Ahmet Inan
    TI__Genius 9046 points

    Hi Ahmet,

    Oh that is so sweet! It must be a lovely gift!

    I think you can use below application note illustration for how to solve back EMF. I am not familiar with motor driver IC but maybe you can initiate another E2E post corresponding to motor driver IC and check the feature with them. Thanks!

    https://www.ti.com/lit/pdf/slla527

    -Wenhao

  • 0 in reply to Wenhao Wu
    Intellectual 450 points

    Hi Wenhao,

    Thank you for your guidance. 

    -Ahmet