【ShiShuo Design】The input voltage is too high/low to power the load? Special converters can help!

In some applications, the existing supply voltage can directly drive the load without the use of an additional voltage converter. Sometimes, when an abnormal operating state occurs, the supply voltage may be too high or too low to directly power the load. In these cases, special voltage converters optimized for such operation can be used. For example, an industrial 24 V system is such an application. Let's assume that the load requires a 24 V supply voltage, but the available 24 V input voltage is sometimes stepped up to 38 V or stepped down to 15 V, which is beyond the allowable supply voltage range of the load. For these applications, a typical boost regulator or a buck-boost regulator can be used.

Figure 1 shows a system diagram for this type of application. The portable radio is supplied by a battery. The load can accept voltages between 10 V and 14 V, but the voltage source may have an output range of 8 V to 16 V. An interposed buck-boost regulator can then convert the voltage to 12 V at the output. If the supply voltage is slightly below 12 V, the converter operates in boost mode, and if it is above 12 V, it operates in buck mode.

Figure 1. Allowable voltage range of the load in the system.

Smaller than the possible voltage range of the power supply.

Systems such as the one shown in Figure 1 work well, but there is room for improvement. If the voltage source voltage can directly power the load most of the time, the buck-boost regulator can be used in pass-through mode. In this case, the input voltage is passed directly to the output of the buck-boost regulator within the input voltage range defined by the power supply designer. The advantage of this operation is that there are no switching losses of any kind, and the circuit efficiency is very high. In addition, since no current pulses are generated in this mode of operation, the circuit generates very low electromagnetic radiation when operating.

Figure 2 shows the power stage of the new LT8210 buck-boost controller circuit with pass-through mode. In this mode, the two high-side switches of the H-bridge are permanently on and the two low-side switches are permanently off. In this way, depending on the current and voltage conditions, nearly 100% efficiency can be achieved.

Figure 2. LT8210 buck-boost regulator with pass-through mode.

Used to transmit voltage in standard operation.

In addition to the buck-boost solution (LT8210), boost regulators also support pass-through mode. The new LT8337 Silent Switcher ^® boost regulator from Analog Devices has pass-through mode integrated into it. Figure 3 illustrates the concept behind the LT8337 boost converter. When pass-through mode is enabled, the high-side switch is permanently on and the low-side switch is permanently off.

Figure 3. LT8337 boost regulator with Silent Switcher technology.

A pass-through mode is also available.

In a boost regulator, high-side switching is usually performed through a flyback diode. This way, the regulator supply voltage that increases above the set output voltage automatically passes through the inductor and flyback diode. However, a dedicated pass-through mode helps effectively reduce the voltage drop across the diode by actively turning on the high-side MOSFET. The pass-through mode also takes care of shutting down all unnecessary functions of the LT8337. As a result, the current consumption of the IC itself may only reach 15 µA. This is particularly effective for battery-powered applications.

The pass-through mode can increase the efficiency of the power supply and improve the EMC performance. These advantages are particularly significant for applications where the available supply voltage is usually within the permissible voltage range of the load. However, the user must also be aware that in the pass-through mode, no output voltage regulation occurs within the defined voltage threshold range, although such regulation is not required in many applications.