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LMG2640: High Side of Halfbridge is permanently off and BST Voltage is only 1V and not 5V

Karsten Chrobak
Prodigy 10 points
Part Number: LMG2640


My main problem ist that the High Side DH to SW Mosfet is not switching like INH Pin. I have changed many valus of the connected Capacitator on the BST Pin but nothing is aktivate the High Side Mosfet of the LMG2640. The measured voltage on the BST Pin is 1V everytimes but in the datasheet it should be min 5V, but how? The Low Side SW to SL Mosfet is correct switching by the PWM Signal on INL Pin. 

Perhaps someone has an idee for me.

  • 0
    TI__Expert 7515 points

    Hi Karsten,

    Based on what you have mentioned, it is expected that the LMG2640 high-side FET is not switching because the BST pin is 1V. 

    The BST-SW voltage needs to be typical 7V as shown in the electrical characteristics table:

    Please ensure that whatever method you use to provide the power to high-side (bootstrap, isolated bias supply) is providing enough voltage for the device to be enabled. 

    Thanks,

    John

  • 0 in reply to John Gomez
    Prodigy 10 points

    Thanks for your prompt answerer but I can not find my mistake.

    My BST is not connected to some supply voltage, only connected to the Capacitor 330nF and I Measured BST Voltage by Multimeter.

    But:

    INH = 0V and INL = 0V Both Side Mosfets are closed. BST Voltage = 1V

    INH = 0V and INL = 3,3V Low Side Mosfet switch to on and High Side is closed. BST Voltage = 15V = AUX

    INH = 3,3V and INL = 0V Both Side Mosfets are closed. BST Voltage = 1V

    Why the BST Voltage only 1V and not more? 1V over SW is not enough to switch High Site Mosfet to on and why BST Voltage goes up to AUX Voltage if I set INL to High Level 3,3V? My circuit is on a new PCB Layout in a full bridge circuit and both sides the same result.

    Thanks for help

    Regards Karsten

  • 0 in reply to Karsten Chrobak
    TI__Expert 7515 points

    Hi Karsten, 

    As you know, the bootstrap circuit can only charge the bootstrap capacitor when the low-side FET is on. 

    This is the only way that bootstrap can get charged.  

    Please provide switching waveforms including AUX, SW, BST, INL, VDD. 

    Thanks,

    John

  • 0 in reply to John Gomez
    Prodigy 10 points

    Hi John,

    thanks for your help. Now I have made a paint of my circuit for common understanding. I want to have a simple PWM circuit to drive a pos/neg load at the PWM Input to down side FET_L and the upper side FET_R is permanently switch on. For correct understanding in my case the BST input need an insolated separate power supply of 7V input to SW and not the Capacitor? 

    Regards Karsten

  • 0 in reply to Karsten Chrobak
    TI__Expert 7515 points

    Hi Karsten, 

    No, you do not need an isolated bias supply. You may use the integrated bootstrap rectifier inside the LMG2640. You still need to put a bootstrap capacitor from BST to SW. 

    Since you are mentioning that the measured voltage from BST-SW is only 1V, I am asking for more information from you so that we can identify where the issue is. 

    We need to see the real switching waveforms with an oscilloscope, not from a drawing. 

    Thanks,

    John