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DAC8760: The purpose of capacitor C1 on Figure.87 "Boot mode configuration"

Part Number: DAC8760

Hi,

According to datasheet 8.4.2 Boot configuration for IOUT, we can find the C1 capacitor (Between Iout-pin and GND), and this capacitor’s recommendation vale is 0.22uF.

Could you please let us know the purpose of this capacitor? And, could you please let us know how to calculate the value on this capacitor?

As the actual measurement result, when value of this capacitor is small, the error tends to increase when the output current is set high. My customer is asking it to explain the reason. 

Thanks and best regards,
 M.HATTORI.

  • Hello,

    The capacitor C1 in the boost transistor configuration has two purposes. First it acts as a filter for higher frequency noise for the DAC+ external BJT circuit. Additionally, it helps to stabilize the external boost transistor circuit.

    What capacitor value did the customer change to and how much error is the customer observing when the capacitor is changed?

    Regards

    Reza

  • Hi,

    >>What capacitor value did the customer change to and how much error is the customer observing when the capacitor is changed?

    The evaluation result (4-20mA) is as follows'

    Case-1 : C1=0.22uF
    4mA:Iout= 3.995mA
    12mA:Iout=11.982mA
    20mA:Iout=19.970mA

    Case-2 : C1=220pF
    4mA:Iout= 3.996mA
    12mA:Iout=11.982mA
    20mA:Iout=20.078mA (*)

    When 220pF, 20mA output is set, the transistor gate voltage oscillates at 80mVpp with a period of 150ns.

    "1. Please let us know the reason of this phenomena on Case-2.

     "2. Also, in general, please let us know how to decide this capacitor value.   Cut& Try?? or to calculate it on desk??

    Please advise us.

    Thanks and best regards,
    M.HATTORI.

  • Hello,

    1. As suspected, 220pF is not enough to stabilize the external boost transistor circuit. The observed oscillation is due the instability of the circuit. The measured currents are also 'off' because the dmm is averaging the current oscillations as it measures causing inaccurate results.

    2. The capacitor was determined by simulating the design to ensure stability under all test cases. That is how 0.22uF was determined. I have also tested devices successfully using 22nF but I wouldn't suggest any capacitor values less than 22nF. 

    Regards,

    Reza