JAJSJS0A June 2020 – September 2020 TPS23734
PRODUCTION DATA
Converters require a soft-start to prevent output overshoot on startup. In PoE applications, the PD also needs soft-start to limit its input current at turn on below the limit allocated by the power source equipment (PSE).
For flyback applications using primary-side control, the TPS23734 provides closed loop controlled soft-start, which applies a slowly rising ramp voltage to a second control input of the error amplifier. The lower of the reference input and soft-start ramp controls the error amplifier, allowing the output voltage to rise in a smooth monotonic fashion.
In all other applications where secondary-side regulation is used, the TPS23734 provides a current-loop soft-start, which controls the switching MOSFET peak current by applying a slowly rising ramp voltage to a second PWM control input. The lower of COMP-derived current demand and soft-start ramp controls the PWM comparator. Note that in this case there is usually a (slower) secondary-side soft-start implemented with the typical TL431 or TLV431 error amplifier to complement the action of the primary-side soft-start.
The soft-start period of the TPS23734 is adjustable with a capacitor between SST and RTN. During soft-start, CSST (Figure 9-1) is being charged from less than 0.2 V to 2.45 V by a ~10µA current source. Once VSST has exceeded approximately 2.1 V (VSTUOF), the VCC startup is also turned off.
The actual control range of the primary-side closed-loop soft-start capacitor voltage is between 0.25V and 2V nominally. Therefore, the soft-start capacitor value must be based on this control range and the required soft-start period (tSS) according to:
The actual control range of the current-loop soft-start capacitor voltage is between 0.6V and 1.2V nominally. Therefore, the soft-start capacitor value must be based on this control range and the required soft-start period (tSS) according to:
Note that the VCC startup turns off only when 2.1 V is reached, allowing additional time for the secondary-side soft-start to complete. For more details regarding the secondary-side soft start, refer to Application Information.