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CSD17575Q3: Input Bounce when switching the MOSFET

Kaan
Prodigy 30 points
Part Number: CSD17575Q3

Tool/software:

Hello to all.

I've been trying to design a battery powered LED design circuit. I am using TPS61235 for boosting a 18650 battery paralelled with 2 cells. The LED board consumes 1.3A. I also use BQ29700 for battery protection. Battery voltage goes straight to the boost converter, enables and boosts after waiting necessary time for boost converters precharge. Then the 5.1V powers the circuit. And the MCU gives PWM to the gate/base of the semiconductor switching element. When this happens there is a heared noise coming out from the circuit and the input voltage starts to bounce. 

I increased the frequency of the PWM to 100kHz from 1kHz and the audible noise is lost but even if the frequency is high the input bounces still. This yields the minimum input voltage level of the boost converter not satisfied or battery protection cuts the connection and the LEDs start to work uncontinuously. This happens around the voltage of the battery reduces to around 3.2 - 3.1V. There is still 240mV pk-pk according to the oscilloscope. I tried to add an 1uH inductor and capacitor just before the switching element comes in. (Same inductor with boosting). Nothing changed.

My original scenario was when the battery is discharged to 2.8V the battery protection cuts the battery connection from the circuit and waiting for recharge. In this case I can control the operation with software not by the hardware. I cut the PWM operation in around 3.3V which results insufficient battery capacity usage.

First i tried to design a darlington pair but didn't satisfied. Then replaced FZT with CSD17575Q3 and removed R14,R17,Q9 and shorted NBASEJ with NBASE.

When the load current increases more unstable the circuit gets. So the uncontinuous phase can start even in 3.7V - 3.9V of the battery. 

I tried the load by connecting it to the output of the boost converter everything worked fine even in 2A loads etc the problem occurs only in switching phase.

Any help is highly appreciated

  • 0
    TI__Mastermind 38580 points

    Hello Kaan,

    Thanks for your interest in TI FETs. What is the voltage level of the PWM signal driving the gate of the FET? It must be ≥ 4.5V to ensure the FET is fully on. How much current does the FET need to conduct when it is on? Do you have adequate decoupling capacitors across output of the boost converter placed near the LEDs and FET? Good quality ceramic capacitors placed at the top of the LED string to the source of the FET with minimal loop area may help reduce the bouncing at the output of the boost converter. You should also have decoupling across the input to the boost converter placed near the IC.I believe the datasheet shows a 10μF capacitor. The CSD17575Q3 is a low on resistance, high current FET. Depending on the current requirements, you may be able to use a lower cost/higher resistance FET for this application. Can you share switching waveforms, VGS & VDS?

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    Prodigy 30 points

    Thanks for the reply John appreciated for that

    I think I've found it. The problem is in the operation of the boost converter TPS61235. The output voltage 5.2V instead of 5.1V fixed. That seems no problem at first but as long as it should be fixed voltage level the output is a bit problematic. The sensing topologies inside of the IC is affected due to abnormal operation of the IC. I tried to give the 5.1V from outside of the board and used boost converter for just supplying the MCU therefore switching the FET, it worked as expected. The board shuts itself on 2.7V without any problems. When the load is connected to the board itself probably the sensed output current is much higher than the actual current and it fails to stay under current limit of the boost converter. 

    In answer to your questions; gate of the FET is supplied by the MCU that is supplied by the boost converter itself with satisfying decoupling capacitors around. 

    The FET is needed to conduct 1.3A and yes I have placed capacitors on the LED supply side. Used 1uH inductor for the switching side and a capacitor.

    I agree with the high cost considering the operation of the FET. These are in my inventory left from battery protection FETs. Just tried to see the operation.