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UC2825: About UC2825

takahiko yokota
Mastermind 7135 points
Part Number: UC2825

Hi

Please let me ask you the two questions about UC2825.
1. The behavior of the attached document
2. The Voltage of E/A out terminal start-up switching

Best regards,

Yokota

  • 0
    TI__Mastermind 26215 points
    Hi Takahiko-san,

    The inihibit circuit circled in the attached document is used to override the error amplifier such as during startup (soft-start) when ILIM/SD exceeds 1.4V (protection). The voltage of the E/A out terminal should ramp linearly at startup. The ramp rate will be dependent on your soft start capacitance. The range of the E/A out voltage is 0 to 5V.

    Best regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Mastermind 7135 points

    Hi Ben-san

    Thank you for the reply.

    Could you provide the internal soft start circuit? We are thinking it is below circuit. Is our understanding correct?
    How much is the voltage of E/A Out which the IC starts switching?

    Best regards,

    Yokota

  • 0 in reply to takahiko yokota
    TI__Mastermind 26215 points

    Hi Takahiko-san,

    The only information on the internal soft start I have available is what is shown in the block diagram. It limits the max duty cycle during startup and is a function of the capacitance connected to the soft start pin. As the internal current source charges the soft start capacitance, the maximum allowable duty cycle increases.

    The E/A out voltage at which the device begins switching is dependent on your configuration of the ramp pin.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Mastermind 7135 points
    Hi Ben-san


    Sorry for the many questions.
    There are the following description in SLUA125.

    Note that a well defined time is required between this instant and the time when the first output pulse can next occur.
    The capacitor begins at 0.2 volts and the error amplifier output is internally clamped to the soft start capacitor voltage.
    Back at the PWM comparator, however, there is a 1.25 volt offset on the ramp pin to facilitate zero duty cycle.
    Therefore, the soft start capacitor must charge from 0.2 volts to 1.25 volts before the PWM comparator is active.


    Does this description meets the following section in UC2825's datasheet?

    Oscillator Section (cont.)
    　Ramp Valley : min = 0.7 V typ = 1.0 V max = 1.25 V


    Best regards,
    Yokota
  • 0 in reply to takahiko yokota
    TI__Mastermind 26215 points
    Hi Takahiko-san,

    Yes, the output of the error amplifier must exceed 1.25V to take effect.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Mastermind 7135 points

    Hi Ben-san

    Thank you for the reply.
    Is UC2825 designed to start switching when the SS terminal voltage is 1.25 V or higher?

    Best regards,
    Yokota
  • 0 in reply to takahiko yokota
    TI__Mastermind 26215 points
    Hi Takahiko-san,

    The device can start switching once the SS terminal voltage reaches 1.25 V.

    Best Regards,
    Ben Lough