10.3.1.1 Synchronous Operation

The device uses three internal N-channel MOSFETs to maintain synchronous power conversion at all possible operating conditions. This enables the device to keep high efficiency over a wide input voltage and output power range.

To avoid ground shift problems due to the high currents in the switches, two separate ground pins, GND and PGND, are used. The reference for all control functions is the GND pin. The power switches are connected to PGND. Both grounds must be connected on the PCB at only one point, ideally close to the GND pin. Due to the 3-switch topology, the load is always disconnected from the input during shutdown of the converter.

10.3.1.2 Down Regulation

A boost converter only regulates output voltages which are higher than the input voltage. This device operates differently. For example, it is able to regulate 3 V at the output with two fresh alkaline cells at the input having a total cell voltage of 3.2 V. Another example is powering white LEDs with a forward voltage of 3.6 V from a fully charged Li-Ion cell with an output voltage of 4.2 V. To control these applications properly, a Down Conversion mode is implemented.

If the input voltage reaches or exceeds the output voltage, the converter automatically changes to a Down Conversion mode. In this mode, the control circuit changes the behavior of the two rectifying switches. While continuing switching, it sets the voltage drop across the rectifying switches as high as needed to regulate the output voltage. This means the power losses in the converter increase. This must be taken into account for thermal consideration.

10.3.1.3 Device Enable

The device is put into operation when EN is set high. It is put into Shutdown mode when EN is set to low. In Shutdown mode, the regulator stops switching, all internal control circuitry including the UVLO comparator is switched off, and the load is disconnected from the input. Current does not flow from input to output or from output to input. This also means that the output voltage can drop below the input voltage during shutdown.

10.3.1.4 Softstart and Short-Circuit Protection

During start-up of the converter, the duty cycle and the peak current are limited in order to avoid high peak currents drawn from the battery. After being enabled, the device starts operating. At first, it keeps the main output VOUT disconnected, and charges the capacitor at VAUX. Once the capacitor at VAUX is charged to about 2.5 V, the device switches to normal operation. This means VOUT is turned on and the capacitor at VOUT is charged, while the load connected to the device is supplied. To ramp up the output voltage in a controlled way, the average current limit is set to 400 mA and rises proportional to the increase of the output voltage. 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 does not increase. There is no timer implemented. Thus the output voltage overshoot at startup, 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 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. When there is a short at the output during operation, the current limit is decreased accordingly.

The device can also start into a Prebias on the outputs.

10.3.1.5 Current Limit

The device current limit limits the average current in the inductor. In a boost connector, this is the input current. If an excessive load requires an input current greater than the average current limit, the device limits the input current by reducing the output power delivered. In this case, the output voltage decreases.

10.3.1.6 Undervoltage Lockout

An undervoltage lockout function prevents the main output at VOUT from being supplied if the voltage at the UVLO pin drops below 0.25 V. When using a resistive divider at the voltage to be monitored, for example the supply voltage, any threshold for the monitored voltage can be programmed. If in undervoltage lockout mode, the device still maintains its supply voltage at VAUX, and it is not turned off until EN is programmed low. This undervoltage lockout function is implemented in order to prevent the malfunctioning of the converter.

10.3.1.7 Thermal Shutdown

The device has a built-in temperature sensor which monitors the internal IC temperature. If the temperature exceeds the programmed threshold (see electrical characteristics table), 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 thermal shutdown threshold.