A 4-switch buck-boost regulator can regulate through occasional large amplitude undervoltage and overvoltage transients, but at the price of operating with high quiescent current, lower efficiency, and increased noise in the far more common, nominal conversion region.
Pass-Thru Operating Mode Brings High Efficiency and EMI Performance Buck-Boost Region
The LT8210 is a 4-switch buck-boost dc-to-dc controller that can operate conventionally with a fixed output voltage and also features a new Pass-Thru™ operating mode (Figure 4), which eliminates the switching losses and EMI through a programmable input voltage window. It operates from 2.8 V to 100 V, allowing it to regulate through the most severe battery drop during engine start to the peak amplitude of an unsuppressed load dump. It comes with built-in reverse battery protection to –40 V, which is implemented with the addition of a single N-channel MOSFET (DG in Figure 5).
In Pass-Thru mode, the output voltage is regulated to the edges of a voltage window when the input voltage is outside the window. The top of the window and the bottom are programmed via the FB2 and FB1 resistor dividers. When the input voltage is within this window, the top switches (A and D) turn on continuously, passing the input voltage directly to the output. In this nonswitching state, the total LT8210 quiescent current drops to tens of microamps. The absence of switching means no EMI and no switching losses, enabling efficiencies of greater than 99.9%.
For those who want the best of both worlds, the LT8210 allows transitioning between its different operating modes on the fly by toggling the MODE1 and MODE2 pins. In other words, the LT8210 can operate as a conventional buck-boost regulator with fixed output voltage (in CCM, DCM, or Burst Mode™) in some situations and then change to Pass-Thru mode as the conditions in the application change. This can be a useful feature for always-on systems and in start-stop applications.
The Pass-Thru solution in Figure 5 passes the input to output in the window between 8 V and 17 V. When the input voltage is above the Pass-Thru window, the LT8210 bucks it down to a regulated 17 V output. If the input drops below 8 V, the LT8210 boosts the output to 8 V. As a protection feature, switching operation is triggered within the Pass-Thru window to control current if either the inductor current limit or programmed average current limit (via the IMON pin) are exceeded.