【Simulation Class】Using Split-Rail Method to Extend Boost Converter Input Voltage Range

By: Haifeng Fan, System Engineer, Texas Instruments

introduction

Wide input range DC/DC controllers usually have a built-in undervoltage lockout (UVLO) circuit to prevent converter malfunction when the input voltage is below the UVLO threshold. However, in some applications, the input voltage is above the UVLO threshold at startup but may drop below it later. In the event of a load transient or supercapacitor discharge, the UVLO circuit may cause an unwanted shutdown. In addition, in some applications, the input voltage is always below the UVLO threshold, which generally does not allow these controllers to be used. This article introduces several split-rail methods to extend the boost converter input voltage range so that we can use these controllers with input voltages below their UVLO thresholds. Design examples and test results are provided to verify the effectiveness of these methods. Minimum Input Voltage for Boost Converters Figure 1 shows a typical boost converter with a single input supply (VIN), which provides the input voltage to the power stage and the bias voltage to the controller. The controller minimum bias voltage at the VIN pin is set by the controller's input UVLO threshold. To guarantee the functionality of the high-side current sensing boost converter (see Figure 1a), the minimum input voltage of the power stage is defined by the minimum common-mode voltage of the current sensing comparator. This is because the input voltage is also connected to the non-inverting input of the current sense comparator. The minimum common-mode voltage of the current sense comparator is usually less than the input UVLO threshold of the controller. For a boost converter with low-side current sensing (see Figure 1b), the input voltage of the power stage is not directly connected to the current sense comparator. Therefore, there is no requirement to match the minimum common-mode voltage. Therefore, when using a single-rail structure, the input voltage of the power stage and the bias voltage of the controller are connected together, so that the input UVLO threshold of the controller imposes a limit on how much the input voltage of the boost power stage can drop. Figure 1 Single-rail boost converter

As shown in Figure 2, the input supply of the boost converter can be split into two rails: the power stage input rail (VIN) and the controller's bias input rail (VBIAS). When using a split-rail structure, VIN can drop below the UVLO threshold, although VBIAS is still required to be above the controller's UVLO threshold to turn on the controller. Since VBIAS only needs to provide very small power, it can be generated by a charge pump or even share another existing voltage rail in the system. In this way, the voltage range of the power rail (VIN) can be extended.

Figure 2 Split-rail structure boost converter

This article will introduce several methods to implement this split-rail structure. TI's TPS43061 synchronous boost controller will be used to illustrate the split-rail concept and verify the effectiveness of the methods described. The boost controller has a high-side current sense comparator and an internal input UVLO circuit at the bias supply input (VIN) pin.

Figure 3 shows the turn-off waveform of the boost converter in the single-rail structure shown in Figure 1a. Once VIN drops below 3.9V (the controller's UVLO turn-off threshold), the converter stops switching. The boost converter will only turn on when VIN rises above the 4.1V UVLO turn-on threshold.

Figure 3 Turn-off waveform of the boost converter in the single-rail structure

Extending the Input Voltage Range After Startup

In some applications with only one input power source, the input supply voltage is greater than the controller’s UVLO turn-on threshold at startup. However, after startup, it can drop below the input UVLO threshold, causing an unexpected shutdown. For example, in power systems using photovoltaic panels and supercapacitors as input power sources, discharge can cause the input voltage to drop below the controller’s UVLO turn-off threshold. Another example is a power system driven by a USB power line, where its voltage can drop significantly during load transients, causing an unexpected system shutdown.

For these applications, if VOUT is within the VBIAS specification (which is always greater than the UVLO turn-on threshold), VOUT can be fed back as a bias supply (VBIAS) through a diode. After startup, VBIAS is controlled to VOUT, which is greater than VIN, and remains above the UVLO threshold even when VIN drops below this threshold. As long as VIN can meet the minimum common-mode voltage requirement of the current sense comparator, the boost converter can maintain normal operation.

Figure 4 Split-rail method to feed back VOUT as a bias supply

Figure 5 shows the shutdown waveforms for the boost converter shown in Figure 4, where VOUT is set to 6V and fed back as the bias supply. When VOUT is above VIN after startup, ignoring the diode forward voltage drop, the bias supply voltage (VBIAS) is controlled to be VOUT greater than VIN. Therefore, when VIN drops below 3.9V, VBIAS remains above the 3.9V UVLO shutdown threshold. VOUT remains in regulation until VIN drops below the minimum common-mode voltage of the current sense comparator (1.9V in this case). This means that the minimum input voltage (VIN) after startup drops from 3.9V to 1.9V.

Figure 5 Shutdown waveforms for the structure shown in Figure 4

Extending the Startup Input Voltage Range

Lithium-ion (Li-Ion) batteries are widely used in smartphones, tablets, and other handheld devices. The nominal voltage of a single Li-Ion battery of 3.6V usually has a range of 2.7V to 4.2V due to the need to discharge and recharge. Even before startup, it is lower than the UVLO threshold of some wide input range boost controllers. For these applications, neither the single-rail solution nor the split-rail method of feeding back VOUT as a bias supply will work. We need a separate bias supply different from the battery input.

Fortunately, the bias battery only needs to provide very low power. If there is another power rail in the system that is higher than the existing UVLO turn-on threshold, it can be connected to VBIAS at the same time as the power rail (VIN) is connected to the battery (see Figure 2). If not, a charge pump can be added for the bias supply (see Figure 6).

Figure 6 Split-rail method using a charge pump to generate bias voltage

In this example, since the battery input range is 2.7V to 4.2V, TI’s TPS60150 charge pump generates a regulated 5V supply that is above the UVLO turn-on threshold of the TPS43061 controller so that it can be used as a bias supply. With a charge pump using a split-rail approach, the boost converter can start and run with a single input supply that is above the boost controller’s UVLO turn-on threshold.

Figure 7 shows the startup waveforms for the boost converter shown in Figure 6. Although VIN is only 2.7V, since VBIAS is regulated at 5V, the converter can start and run with a single 2.7V supply. With this split-rail approach, the minimum operating input voltage range of the boost converter is further extended from 4.1V to 2.7V.

Figure 7 Startup waveforms for the structure shown in Figure 6

Conclusion

Boost converters typically require two inputs to operate: the power stage input supply and the controller bias supply. The controller’s UVLO threshold determines the lower limit of the bias supply. In addition, it also limits the input supply to the power stage if the two rails are tied together to share a common input supply. The split-rail approach separates the power rail from the bias rail to eliminate the limitation on the minimum operating voltage of the power rail. This can extend the input voltage range of the boost converter.