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LM5576 Vcc and Power Dissipation

Ahmad Qadomi
Prodigy 80 points
Other Parts Discussed in Thread: LM5576

I have a question not related to the topic directly but it's related to the Chip LM5576. In datasheet it's mentioned that VCC could be supplied by an external voltage in the range 7.5-14V which reduces internal power dissipation. Also in Electrical Characteristics I can see that the max Bias current is 4.5mA. So, can i use the input voltage if it has a low regulation (48-50v) and a voltage divider resistors to supply VCC? and if I did that is there an observable difference in thermal characteristics and power dissipation? does it worth it?

Thanks,

  • 0
    TI__Mastermind 19891 points

    Hi Ahmad, 

    Thank you for the kind words. I split the previous thread and copied your LM5576 question to this new thread with a new title. It would help others if they try to search similar topics.

    Let me try to answer your question. 

    The approach you describe will take the power dissipation of the VCC regulator and put it outside of the package, but using resistor divider will not make sense because you will have to dissipate quite a bit of power in the divider in order to get the Vcc voltage range (when the part is switching and not switching). If you are going to use external Vcc, it is better to derive the Vcc voltage from some other external rail or external regulator. 

    You can run some calculations to figure out if it makes sense. One thing to consider is the total Vcc current. Here are some details.

    The Vcc current (call it Ivcc) is the sum of the bias current for the chip (I will call it Ibias) and the gate charge current to drive the power MOSFET inside (I will call this Igate). The spec you are referring to is only Ibias. We need to also consider the Igate portion. When the regulator is switching, the portion of power dissipated in the internal VCC regulator is (Vin - Vcc) x Ivcc, where the Ivcc = Igate + Ibias.  The gate charge current Igate depends on the chosen switching frequency. I have taken some measurements in the past of the total Ivcc current (Igate + Ibias) vs the switching frequency. Here is some data:

    Frequency
    (kHz)    		 I vcc
    (mA)
    200 6.1
    300 7.8
    400 9.6
    500 10.9

    The relationship is linear so if you switch at some other frequency you can estimate the Ivcc between the data points.

    If you are stepping down from 50V input to, let's say 5V output and switch at 500kHz, you would dissipate 490mW just in the Vcc regulator. With thermal resistance of 30degC/W, this is about 15degC difference in the junction temperature if this loss was outside of the package. 

    What input and output voltage, maximum ambient temperature, and board area you are considering?

    Regards, 
    Denislav