7.3.1 Device Enable

The device is put into operation when EN is set high. It is put into a shutdown mode when EN is set to GND. In shutdown mode, the regulator stops switching, all internal control circuitry is switched off, and the load is disconnected from the input. This also means that the output voltage can drop below the input voltage during shutdown. During start-up of the converter, the duty cycle and the peak current are limited in order to avoid high peak currents flowing from the input.

7.3.2 Undervoltage Lockout

An undervoltage lockout function prevents device start-up if the supply voltage at VINA is lower than approximately its threshold (see Electrical Characteristics ). When in operation, the device automatically enters the shutdown mode if the voltage at VINA drops below the undervoltage lockout threshold. The device automatically restarts if the input voltage recovers to the minimum operating input voltage.

7.3.3 Overtemperature Protection

The device has a built-in temperature sensor which monitors the internal IC temperature. If the temperature exceeds the programmed threshold (see Electrical Characteristics ) the device stops operating. As soon as the IC temperature has decreased below the programmed threshold, it starts operating again. There is a built-in hysteresis to avoid unstable operation at IC temperatures at the overtemperature threshold.

7.4.1 Soft-Start and Short Circuit Protection

After being enabled, the device starts operating. The average current limit ramps up from an initial 400 mA following the output voltage increasing. At an output voltage of about 1.2 V, the current limit is at its nominal value. If the output voltage does not increase, the current limit will not increase. There is no timer implemented. Thus the output voltage overshoot at start-up, as well as the inrush current, is kept at a minimum. The device ramps up the output voltage in a controlled manner even if a very large capacitor is connected at the output. When the output voltage does not increase above 1.2 V, the device assumes a short circuit at the output and keeps the current limit low to protect itself and the application. At a short at the output during operation the current limit also will be decreased accordingly. At 0 V at the output, for example, the output current will not exceed about 400 mA.

7.4.2 Buck-Boost Operation

To regulate the output voltage properly at all possible input voltage conditions, the device automatically switches from step-down operation to boost operation and back as required by the configuration. It always uses one active switch, one rectifying switch, one switch permanently on, and one switch permanently off. Therefore, it operates as a step-down converter (buck) when the input voltage is higher than the output voltage, and as a boost converter when the input voltage is lower than the output voltage. There is no mode of operation in which all 4 switches are permanently switching. Controlling the switches this way allows the converter to maintain high efficiency at the most important point of operation; when input voltage is close to the output voltage. The RMS current through the switches and the inductor is kept at a minimum, to minimize switching and conduction losses. Switching losses are also kept low by using only one active and one passive switch. For the remaining 2 switches, one is kept permanently on and the other is kept permanently off, thus causing no switching losses.

7.4.3 Power-Save Mode and Synchronization

The PS/SYNC pin can be used to select different operation modes. To enable power-save mode, PS/SYNC must be set low. Power-save mode is used to improve efficiency at light load. If power-save mode is enabled, the converter stops operating if the average inductor current gets lower than about 300 mA and the output voltage is at or above its nominal value. If the output voltage decreases below its nominal value, the device ramps up the output voltage again by starting operation using a programmed average inductor current higher than required by the current load condition. Operation can last for one or several pulses. The converter again stops operating once the conditions for stopping operation are met again.

The power-save mode can be disabled by programming high at the PS/SYNC. Connecting a clock signal at PS/SYNC forces the device to synchronize to the connected clock frequency. Synchronization is done by a phase-locked loop (PLL), so synchronizing to lower and higher frequencies compared to the internal clock works without any issues. The PLL can also tolerate missing clock pulses without the converter malfunctioning. The PS/SYNC input supports standard logic thresholds.