7.3.1 Auto-Skip Eco-Mode™ Control

The TPS54628 is designed with Auto-Skip Eco-Mode™ to increase light load efficiency. As the output current decreases from heavy load condition, the inductor current is also reduced and eventually comes to point that its rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous conduction modes. The rectifying MOSFET is turned off when its zero inductor current is detected. As the load current further decreases the converter run into discontinuous conduction mode. The on time is kept almost the same as is was in the continuous conduction mode so that it takes longer time to discharge the output capacitor with smaller load current to the level of the reference voltage. The transition point to the light load operation I_OUT(LL) current can be calculated in Equation 1.

Equation 1.

7.3.2 Soft Start and Prebiased Soft Start

The soft-start function is adjustable. When the EN pin becomes high, 6-µA current begins charging the capacitor which is connected from the SS pin to GND. Smooth control of the output voltage is maintained during start-up. The equation for the slow-start time is shown in Equation 2. VFB voltage is 0.765 V and SS pin source current is 6 µA.

Equation 2.

The TPS54628 contains a unique circuit to prevent current from being pulled from the output during start-up if the output is prebiased. When the soft-start commands a voltage higher than the prebias level (internal soft start becomes greater than feedback voltage V_FB), the controller slowly activates synchronous rectification by starting the first low-side FET gate driver pulses with a narrow on time. It then increments that on time on a cycle-by-cycle basis until it coincides with the time dictated by (1–D), where D is the duty cycle of the converter. This scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (V_O) starts and ramps up smoothly into regulation and the control loop is given time to transition from prebiased start-up to normal mode operation.

7.3.3 Output Discharge Control

TPS54628 discharges the output when EN is low, or the controller is turned off by the UVLO protection. The internal low-side MOSFET is not turned on for the output discharge operation to avoid the possibility of causing negative voltage at the output.

7.3.4 Current Protection

The output overcurrent protection (OCP) is implemented using a cycle-by-cycle valley detect control circuit. The switch current is monitored by measuring the low-side FET switch voltage between the SW pin and GND. This voltage is proportional to the switch current. To improve accuracy, the voltage sensing is temperature compensated.

During the on time of the high-side FET switch, the switch current increases at a linear rate determined by V_IN, V_OUT, the on time and the output inductor value. During the on time of the low-side FET switch, this current decreases linearly. The average value of the switch current is the load current I_OUT. The TPS54628 constantly monitors the low-side FET switch voltage, which is proportional to the switch current, during the low-side on time. If the measured voltage is above the voltage proportional to the current limit, an internal counter is incremented per each SW cycle and the converter maintains the low-side switch on until the measured voltage is below the voltage corresponding to the current limit at which time the switching cycle is terminated and a new switching cycle begins. In subsequent switching cycles, the on time is set to a fixed value and the current is monitored in the same manner. If the overcurrent condition exists for 7 consecutive switching cycles, the internal OCL threshold is set to a lower level, reducing the available output current. When a switching cycle occurs where the switch current is not above the lower OCL threshold, the counter is reset and the OCL limit is returned to the higher value.

There are some important considerations for this type of overcurrent protection. The peak current is the average load current plus one half of the peak-to-peak inductor current. The valley current is the average load current minus one half of the peak-to-peak inductor current. Because the valley current is used to detect the overcurrent threshold, the load current is higher than the overcurrent threshold. Also, when the current is being limited, the output voltage tends to fall. When the VFB voltage becomes lower than 65% of the target voltage, the UVP comparator detects it. If the undervoltage condition persists for 250 µs, the device shuts down and restarts in hiccup mode after 7 times the SS period. When the overcurrent condition is removed, the output voltage returns to the regulated value. This protection is non-latching.

7.3.5 Overvoltage Protection

TPS54628 detects overvoltage and undervoltage conditions by monitoring the feedback voltage (VFB). This function is enabled after approximately 1.7 times the soft-start time. When the feedback voltage becomes higher than 125% of the target voltage, the OVP comparator output goes high and both the high-side MOSFET driver and the low-side MOSFET driver turn off. This function is non-latch operation.

7.3.6 UVLO Protection

Undervoltage lockout protection (UVLO) monitors the voltage of the V_REG5 pin. When the V_REG5 voltage is lower than UVLO threshold voltage, the TPS54628 is shut off. This protection is non-latching.

7.3.7 Thermal Shutdown

TPS54628 monitors the temperature of itself. If the temperature exceeds the threshold value (typically 165°C), the device is shut off. This is non-latch protection.

7.4.1 PWM Operation

The main control loop of the TPS54628 is an adaptive on-time pulse width modulation (PWM) controller that supports a proprietary D-CAP2™ mode control. D-CAP2™ mode control combines constant on-time control with an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with both low-ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output.

At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one-shot timer expires. This one shot is set by the converter input voltage, VIN, and the output voltage, VO, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the ESR-induced output ripple from D-CAP2™ mode control.

7.4.2 PWM Frequency and Adaptive On-Time Control

TPS54628 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The TPS54628 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to set the on-time one-shot timer. The on time is inversely proportional to the input voltage and proportional to the output voltage; therefore, when the duty ratio is VOUT/VIN, the frequency is constant.