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TPS61088: Voltage drop at the output when the load current exceeds ~550mA

Rutvik N
Prodigy 40 points
Part Number: TPS61088

Hi,

Here are the design requirements and configuration of the TPS61088 IC in my PCB

  • Supply Source: 2 x Lithium Ion Batteries (in parallel)
  • Output configuration: 7.5V and 2A 
  • Operating Mode: PWM
  • Inductor used: Bao Cheng Elec BCIHP0718-1R0M (1uH)   Saturation current: 11.5A
  • EN pin is pulled high (4.1V)

I am testing the output capabilities of the TPS circuitry using 2 x 10 Ohms, 7 Watt load resistors connected in parallel to the output of the TPS61088 IC.

From the tests I have noticed when a single load resistor is connected to the output, the TPS IC is able to source the required current (~780mA) but when a second load resistor is added at the output in parallel, there is a >0.50V voltage drop. Here is the schematic diagram of the TPS60188 circuitry. The values in black are the ones that are soldered on the board.

Test notes,

LOAD: One 10 Ohm, 7W Load resistance

  • Current consumption at output - 713mA
  • Voltage at output when load is connected -  7.51V
  • Voltage across VIN (C610) pin of TPS61088 IC when load is connected - 3.08V
  • Voltage across VCC (C604) pin of TPS61088 IC when load is connected - 5.51V

LOAD: 2 x  10 Ohm, 7W Load resistance 

  • Current consumption at output - 1030mA
  • Voltage at output when load is connected - 5.36V
  • Voltage across VIN (C610) pin of TPS61088 IC when load is connected - 3.06V
  • Voltage across VCC (C604) pin of TPS61088 IC when load is connected - 5.20V

UPDATE:

After going through a few posts about the TPS61088 IC, I reconfigured the output voltage of the TPS61088 IC to 5.98V (Changed R602 to 300K and R606 to 73K) and tried the load tests again.

But there still is a voltage drop at higher loads (>550mA)

LOAD: One 10 Ohm, 7W Load resistance

  • Current consumption at output - 550mA
  • Voltage at output when load is connected - 5.97V 
  • Voltage at VIN (C610) pin of TPS61088 IC when load is connected - 3.3V
  • Voltage at VCC (C604) pin of TPS61088 IC when load is connected - 5.3V

LOAD: 2 x  10 Ohm, 7W Load resistance

  • Current consumption at output - 1.042A
  • Voltage at output when load is connected - 5.48V
  • Voltage at VIN (C610) pin of TPS61088 IC when load is connected - 2.77V
  • Voltage at VCC (C604) pin of TPS61088 IC when load is connected - 5.23V

Any help will be greatly appreciated. Thank you.

Regards,

Rutvik

  • 0
    TI__Mastermind 40080 points

    Dear Sir,

    How did you measure the Vin, Vout? The Vin, Vout should be measured at the input cap and the output cap which is very close to the IC.  Because there will be voltage drop across the input and the output wires.

  • 0 in reply to Helen chen
    Prodigy 40 points

    Hi Helen,

    Thank you for the quick response.

    I re-soldered the resistors connected to FB Pin i.e, 73K at R606 and 300K at R602 for 5.98V output and did the tests again. As suggested by you, this time I measured the voltages at the input and output capacitors. 

    Supply voltage to the IC is 4.1V (through a regulated power supply unit)

    LOAD: 1 x 10 Ohm, 7W Load resistance

    • Current consumption at output - 569mA
    • Voltage across Vin (C610) of TPS61088 IC when load is connected - 3.46V
    • Voltage across Vcc (C604) of TPS61088 IC when load is connected - 5.39V
    • Voltage across Vout (C602) of TPS61088 IC when load is connected - 6.04V

    LOAD: 2 x  10 Ohm, 7W Load resistance 

    • Current consumption at output - 1047mA
    • Voltage across Vin (C610) pin of TPS61088 IC when load is connected - 2.92V
    • Voltage across Vcc (C604) pin of TPS61088 IC when load is connected - 4.47V 
    • Voltage across Vout (C602) of TPS61088 IC when load is connected - 5.99V

    LOAD: 3 x  10 Ohm, 7W Load resistance  

    • Current consumption at output - 1203mA
    • Voltage across Vin (C610) pin of TPS61088 IC when load is connected - 3.04V
    • Voltage across Vcc (C604) pin of TPS61088 IC when load is connected - 4.50V
    • Voltage across Vout (C602) of TPS61088 IC when load is connected - 4.78V

    Issues:

    1. Voltage drop after the 3rd load resistor is connected

    2. The IC starts to heat up a few seconds after the 2nd load resistor is connected. 

     

    Thanks

  • 0 in reply to Rutvik N
    Prodigy 40 points

    The original design is for 7.8V@2A output based on our application's requirements. And that is what I want to resolve.

    But the 6V output configuration also does not seem to work 

  • 0 in reply to Rutvik N
    TI__Mastermind 40080 points

    Thanks for the update.

    What is the Vout at the no load condition?  Please check the soldering of the Ilim pin and the COMP pin. If the soldering is good, you can detect out a diode from the GND to the Ilim pin, from the GND to the COMP pin.

  • 0 in reply to Helen chen
    Prodigy 40 points

    Hi Helen,

    Vout when there is no load: 5.93V  (IC output configured for the same)
    Drop across ILIM Pin: 1.18V
    Drop across COMP Pin: 1.62V

    I even tried increasing the switching frequency from 646kHz to 2.2MHz to see if that helps but the issue is still exists.

    Regards,
    Rutvik

  • 0 in reply to Rutvik N
    TI__Mastermind 40080 points

    Thanks for the update.

    Please change the compensation network to 10k/4.7nF or 10k/10nF to check whether the output is normal.

    And also please check whether the soldering of each pin is good or not. (There's an anti-paralleled diode from the GND to each pin, you can test out the diode if the pin is well connected )