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LMH6503: LMH6503 IC temp. rise issue while no input...

Peter Tseng
Mastermind 6560 points

Part Number: LMH6503
Other Parts Discussed in Thread: LM27762, THS4541

Tool/software:

Hi Sir,

The customer found the LMH6503 IC will get hot after power on with no input signal!

The power supply provides 4.3V as a power source for LMH6503 and boosts IC(LM27762). The current consumption is 60mA, as shown on the power supply.

The LMH6503 temp. will increase around 7~8 degreeC and boost IC temp. increase around 5 degreeC after 2mins later.

Wait for the system to stabilize and measure the AGC output(red circle) and the waveform as attached.

The increased temp. will narrow the pulse width to around 6ns.

Please help to check if there is anything wrong with the design and any advice is appreciated!

Thanks. 

  • 0
    TI__Mastermind 24524 points

    Hello Peter,

    What is your signal on the VG pin?

    Also, what is the quiescent current (IQ) of the LM6503 when the IC is powered on with no load?  The datasheet showcases 37mA, if you observe a very different IQ you may have an issue with your IC.

    The IC has the thermal resistance values in the operating characteristics table on the datasheet; the R_thetaJA (junction to ambient) is quite high for this device.  It will not release heat to the ambient air effectively, causing the device to heat more easily than a thermally conductive package.  

    You can look at the VG (gain control) bias drift and bias current drift over temperature specs.  

    I am also curious what you are using to set/drive the VG pin through the 10kOhm resistor.

    If IQ is okay the part is powering up correctly.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Mastermind 6560 points

    Hi Sir,

    The VG is pull low.

  • 0 in reply to Peter Tseng
    TI__Mastermind 24524 points

    Hello Peter,

    Thank you for confirming VG is low for this circuit.

    Have you been able to verify correct operating quiescent current?

    Best,

    Alec

  • 0 in reply to Peter Tseng
    TI__Mastermind 24524 points

    Hello Peter,

    Thank you for confirming VG is low for this circuit.

    Have you been able to verify correct operating quiescent current?

    Best,

    Alec

  • 0 in reply to Peter Tseng
    TI__Mastermind 24524 points

    Hello Peter,

    Thank you for confirming VG is low for this circuit.

    Have you been able to verify correct operating quiescent current?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Mastermind 6560 points

    Hi Sir,

    Reply for your question as below.

    What is the quiescent current (IQ) of the LM6503 when the IC is powered on with no load? 

    With no load ,use +-5V to Vin

    IQ measure as below

    +5V: 30mA

    -5V:  30mA

     The datasheet showcases 37mA, if you observe a very different IQ you may have an issue with your IC.

     Not sure datasheet showcases 37mA means total power consumption or each Vin.

     The IC has the thermal resistance values in the operating characteristics table on the datasheet; the R_thetaJA (junction to ambient) is quite high for this device. 

    It will not release heat to the ambient air effectively, causing the device to heat more easily than a thermally conductive package.  

     Our signal input is 250kHZ ,and LMH6503 surface temperature can reach 60°C.

    According R_theta_JA, it’s seems might be normal?

     You can look at the VG (gain control) bias drift and bias current drift over temperature specs.  

     Does any function can compensate this drift?

     I am also curious what you are using to set/drive the VG pin through the 10kOhm resistor.

     Just want to use other analog switch to control gain of VG, between 4 and 10.

     If IQ is okay the part is powering up correctly.

    For other questions, the customer also used the same design but THS4541 instead of LMH6503, with the same amplification(4x4 times).

    They found the the width of LMH6503 is wider than THS4541, and the pulse falling seems damping.

    What is the reason for this?

    And how fast of LMH6503 for gain adjustment? Any other suggestion of AGC but less heat increasment?

    Thanks.

  • 0 in reply to Peter Tseng
    TI__Mastermind 24524 points

    Hello Peter,

    Thank you for the response.  I will take a look at this tomorrow for you.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Mastermind 24524 points

    Hello Peter,

    Has your customer provided any updates on their debugging or analysis?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Mastermind 6560 points

    Hi Alec,

    Cusotmer did not have update!

    How about the last questions/requests that you checked for?

    Thanks.