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LM5156: How should i Calculate Input capacitor, of Output hold on for 10mS

Aditya Yedur
Prodigy 140 points
Part Number: LM5156

Hello Team 

 I am designing Isolated DC to DC converter using LM5156 for below Specification.

Input - 17 to 32 VDC

Output - 5V

Output Current: 3.5A

I need solution for below Condition.

Condition: When my input voltage is below 16VDC at that time My controller needs to turn off and I want to achieve Output voltage hold on for 10ms.

for this condition any calculation available?

Please suggest how should I achieve. 

Thanks

Aditya 

  • 0
    TI__Mastermind 30030 points

    Hi Aditya,

    The LM5156 device has a undervoltage lockout feature (UVLO).
    You can put a feedback divider from your supply to the UVLO pin to trigger an undervoltage lockout once the input voltage drops below 16V.
    (The values for this can be calculated using the LM5156 quickstart calculator available on the product page)

    In UVLO state, the device will be inactive and not switching.
    The device will ramp up again in softstart once the supply voltage rises above the UVLO threshold (+hysteresis).

    Can you give more details on the desired output voltage hold on?
    Should the output be still supplied through the output caps, and will this be with or without active load?

    Thanks and best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 140 points

    Hii Niklas

    Thank You for Reply

    1)   we Need Hold on with Active load please find below Information for hold on.  

    PSU Start voltage: 17V DC.

    Stop Voltage - 10V DC.

    Output wattage - 17.5W (5V 3.5A)

    With this specification how should I calculate Capacitor value?

    Please correct me if i wrong. For HOLD On We need to calculate Input Capacitor Right?

    2)  Second Point How should i Use OVP IN Secondary-Side Regulated Isolated Flyback?

    I want to use OVP In my design.

    Thank You

    Aditya 

  • 0 in reply to Aditya Yedur
    TI__Mastermind 30030 points

    Hi Aditya,

    Thanks for the clarification.

    1.
    The output capacitance depends on how the voltage is allowed to drop during the hold period.
    The common equations for capacitor discharge should apply in this case, so you will find a lot of resources on this online.
    Equation for capacitor discharge:
    Vc = Vo * e^-t/RC

    The output capacitance is essential here, as this capacitance will supply the output if the controller shuts down.

    2.
    As the FB pin is connected to GND in isolated flyback topology, the OVP is inactive and cannot be used. This also applies for the internal softstart.
    For the softstart, an external circuit can be used instead:
    https://www.ti.com/lit/an/snvaa44/snvaa44.pdf
    Unfortunately, we do not have any reference for building an external circuit to detect OVP externally.

    Best regards,
    Niklas