7.4.1 Startup Operation

The device includes a high-voltage current source connected between the HVIN pin and the internal supply for the regulator. When the voltage on the HVIN pin rises, the current source is activated and starts to supply current to the internal 5-V regulator. The 5-V regulator charges the external capacitor connected between VDD pin and GND pin. When the VDD voltage exceeds the VDD turn on threshold (V_VDD(on)) device starts operations. The minimum voltage across HVIN and GND pins to ensure enough current to charge the capacitance on VDD pin is V_HVIN(min). At the First switching cycle the minimum MOSFET off time is set to be > 100 μs and cycle-by-cycle is progressively reduced up to t_OFF(min) providing soft start.

7.4.2 Feedback and Voltage Control Loop

The feedback circuit consists of a voltage comparator with the positive input connected to an internal reference voltage (referenced to GND) and the negative input connected to FB pin. When the feedback voltage at the FB pin is below the reference voltage V_{FB_TH} logic high is generated at the comparator output. This logic high triggers the PWM controller, which generates the PWM signal turning on the MOSFET. When the feedback voltage at the FB pin is above the reference voltage, it indicates that the output voltage of the converter is above the targeted output voltage set by the external feedback circuitry and MOSFET cannot be turned on.

7.4.3 PWM Controller

The PWM controller's input comes from the feedback comparator. When the feedback comparator output goes high, the PWM controller is triggered. This starts the ON-time generation. The ON-time pulse is defined as t_ON(max) and at the end of every ON-time pulse an OFF-time (logic low) pulse starts. The OFF-time pulse is long as t_OFF(min) during converter normal operation, it can be longer up to t_OFF(ovl) at start up or if risk of inductor current runaway is detected. The output of the PWM controller is the PWM signal (See Figure 11). After the OFF-time pulse has ended, the PWM controller can again receive the feedback comparator’s output. If the feedback comparator’s output is still high, then a new ON-time pulse followed by an OFF-time pulse is generated for the PWM signal. If the feedback comparator’s output is low at the point the OFF-time has ended, then the PWM controller continues to output a logic low signal at the output (the PWM signal). The PWM signal stays low until the feedback comparator output goes high, at which point the PWM controller is again triggered, and the ON-time + OFF-time pulse is generated.

The PWM controller does not need a clock signal, and the PWM signal is load dependent. The PWM signal’s maximum frequency is set to f_SW(max) = (1/(t_ON(max) + t_OFF(min))) which occurs when the voltage on the FB pin is continuously below V_{FB_TH}.

Figure 11. UCC28880 Timing Diagram

7.4.4 Current Limit

The current limit circuit senses the current through the power MOSFET. The sensing circuit is located between the source of the power MOSFET and the GND pin. When the current in the power MOSFET exceeds the threshold I_LIMIT, the internal current limit signal goes high, which sets the internal RSTN signal low. This disables the power MOSFET by driving its gate low. The current limit signal is set back low after the falling edge of the PWM signal. After the rising edge of the GATE signal, there is a blanking time. During this blanking time, the current limit signal cannot go high.

7.4.5 Inductor Current Runaway Protection

To protect the device from overload conditions, including a short circuit at the output, the PWM controller incorporates a protection feature which prevents the inductor current from runaway. When the output is shorted the inductor demagnetization is very slow, low di/dt, and when the next switching cycle starts energy stored in the inductance is still high. After the MOSFET switches on, the current starts to rise from pre-existing DC value and reaches the current-limit value in a short duration of time. Because of the intrinsic minimum on-time of the device the MOSFET on-time cannot be lower than t_MIN, in an overload or output short circuit the energy inductance is not discharged sufficiently during MOSFET off-time, it is possible to lose control of the current leading to a runaway of the inductor current. To avoid this, if the ON time is less than t_{ON_TO} (t_{ON_TO} is a device internal time out), the controller increases the MOSFET OFF time (t_OFF). If the MOSFET ON-time is longer than t_{ON_OL} then t_OFF is decreased. The controller increases t_OFF, cycle-by-cycle, through discrete steps until the ON-time continues to stay below t_{ON_TO}. The t_OFF is increased up to t_OFF(ovl) after that, if teh ON-time is still below t_{ON_OL} the off time is kept equal to t_OFF(ovl). The controller decreases t_OFF cycle-by-cycle until the ON-time continues to stay above t_{ON_TO} up to t_OFF(min). This mechanism prevents control loss of the inductor current and prevents over stress of the MOSFET (see typical waveforms in Figure 12 and Figure 13). At start up, the t_OFF is set to t_OFF(ovl) and reduced cycle-by-cycle (if the ON time is longer than t_{ON_TO}) up to t_OFF(min) providing a soft start for the power stage.

Figure 12. Current Runaway Protection Logic Timing Diagram

Figure 13. Current Runaway Protection, Inductor and MOSFET Current

A minimal value needs to be imposed on the inductance value to avoid nuisance tripping of the protection feature that prevents the loss of control of the inductor current. Inadvertent operation of the protection feature limits the output-power capability of the converter. This condition depends on the converter's maximum input operating voltage and temperature. Use Equation 1 to calculate your minimum inductance value.

Equation 1.

The value of Equation 1 can be found by characterization graph of Figure 10.

If the inductance value is too low, such that the MOSFET on-time is always less than t_{ON_TO} timeout and the device progressively increases the MOSFET off-time up to t_OFF(ovl), the output power is reduced and the converter fails to supply the load.

7.4.6 Over-Temperature Protection

If the junction temperature rises above T_J(stop), the over-temperature protection is triggered. This disables the power MOSFET switching. To re-enable the switching of the MOSFET the junction temperature has to fall by T_J(hyst) below the T_J(stop) where the device moves out of over temperature.