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LM70880-Q1: Inquiry regarding design

JH Shin
Guru 10890 points
Part Number: LM70880-Q1
Other Parts Discussed in Thread: LM70880

Tool/software:

Hello,

My customer has some questions about his DC/DC design using the LM70880-Q1.

Q1. They want to use LM70880-Q1 for below 10V/10A. Will it be ok to parallel it with other DC/DC controller?

Q2. When the output voltage is 30V, how should they set the bias voltage? Should they design it to satisfy the withstand voltage of bias to GND by configuring Zener 4.7V in series?

Q3. In the figure below, what is the voltage difference between input and output for the LDO mode operation? Could you explain each case separately when the input voltage is 48V and the output voltage is 30V and 10V?

Q4. Please tell us how to connect PG/Syncount (Can they be connected with 30V output and pull-up resistor 100k resistor?)

Thank you.

JH

  • 0
    TI__Genius 11245 points

    Hi JH,

    Please see my comments below:

    JH Shin said:
    Q1. They want to use LM70880-Q1 for below 10V/10A. Will it be ok to parallel it with other DC/DC controller?

    I am not sure if we can call this paralleling. The outputs and GNDs are different - only the source is common. As long as the source can provide the required power - I don't foresee any issues. 

    LM70880-Q1 is an 8A device - You should look at https://www.ti.com/product/LM706A0-Q1. 

    JH Shin said:
    Q2. When the output voltage is 30V, how should they set the bias voltage? Should they design it to satisfy the withstand voltage of bias to GND by configuring Zener 4.7V in series?

    I am not sure where you want to add a Zener - The bias pin is an input to an internal voltage regulator which regulates the Vcc voltage. You can tie the pin to Vout and use a 4.7uF cap. 

    JH Shin said:
    Q3. In the figure below, what is the voltage difference between input and output for the LDO mode operation? Could you explain each case separately when the input voltage is 48V and the output voltage is 30V and 10V?

    Low Drop-out mode happens when the Vin drops and comes closer to the configured output voltage. So in your case - if input is 48V - the device will work normally for 30V and 10V outputs. For the 30V case - if the 48V falls towards 30V - the device will start getting into Low Drop-out mode (to regulate Vout) - if it falls further (below 30V) - the Vout tracks Vin(with 99% duty cycle). Section 7.3.1 explains this in detail. 

    JH Shin said:

    Q4. Please tell us how to connect PG/Syncount (Can they be connected with 30V output and pull-up resistor 100k resistor?)

    '

    PG/SYNCOUT has a max voltage of 6.5V - it is recommended you pull up this pin to VDDA.

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Guru 10890 points

    Hi Taru,

    Thank you for your kind reply.

    Taru Kapoor said:
    I am not sure if we can call this paralleling. The outputs and GNDs are different - only the source is common. As long as the source can provide the required power - I don't foresee any issues. 

    Sorry for the confusion. The LM70880-Q1 is specified as “Dual-phase operation enabling output currents up to 16A”, which means that the LM70880 will be operated in a parallel configuration in a 10V/10A product. Is it okay to use the LM70880-Q1 in a parallel configuration in the above application? Will the current sharing, etc. be okay?

    Taru Kapoor said:
    I am not sure where you want to add a Zener - The bias pin is an input to an internal voltage regulator which regulates the Vcc voltage. You can tie the pin to Vout and use a 4.7uF cap. 

    They saw that the output voltage should be 30V and the absolute voltage should not exceed 32V, and if it exceeds, it should be tied to GND. So I asked if it would be a good idea to apply a 10V zener diode in series between the output voltage (30V) and the Bias terminal to form a 20V voltage at the bias pin. Can they tie Bias and GND together under the condition of 30V output voltage?

    Thanks,

    JH

  • 0 in reply to JH Shin
    TI__Genius 11245 points

    Hi JH,

    JH Shin said:
    he LM70880-Q1 is specified as “Dual-phase operation enabling output currents up to 16A”, which means that the LM70880 will be operated in a parallel configuration in a 10V/10A product. Is it okay to use the LM70880-Q1 in a parallel configuration in the above application? Will the current sharing, etc. be okay?

    Yes, you can use this configuration. I do not foresee any issues. Please implement your design based on the recommendations in the datasheet (See 7.3.15 Single-Output Dual-Phase Operation in the datasheet). The devices work in an interleaved manner. 

    As mentioned before, we also have a 10A device https://www.ti.com/product/LM706A0-Q1. You can take a look at that as well. 

    JH Shin said:
    They saw that the output voltage should be 30V and the absolute voltage should not exceed 32V, and if it exceeds, it should be tied to GND. So I asked if it would be a good idea to apply a 10V zener diode in series between the output voltage (30V) and the Bias terminal to form a 20V voltage at the bias pin. Can they tie Bias and GND together under the condition of 30V output voltage?

    I guess you can do that - however, the power loss that would have happened in the device gets shifted to another passive and you aren't necessarily gaining any efficiency advantage in the design AND you are adding one extra passive. If the customer sees that the BIAS Pin can exceed 32V during normal operation - you are much better off tying it to GND. 

    Thanks,

    Best Regards,

    Taru