The LT8672 controller features a minimum input operating voltage of 3 V VBATT, enabling the active rectifier to operate through the cold crank pulse with a minimum drop (20 mV) between input and output. Downstream power supplies during a cold crank event see an input voltage no lower than 3 V. This allows use of a buck regulator with a minimum operating voltage of 3 V and low dropout characteristics, such as the LT8650S, to generate a 3 V supply.
Like the LT8650S, many ADI Power by Linear automotive ICs feature minimum input voltage rating of 3 V.
Figure 9 shows the comparison of 1.8 V power supply with the LT8672 and with a traditional diode. The step-down regulator works down to 3 V. As shown, with a traditional diode, VIN to the buck regulator drops to near 2.7 V when the battery voltage VBATT drops to 3.2 V, due to high voltage drop of the diode, triggering the UVLO shutdown of the downstream switching regulator, and its 1.8 V output collapses. In contrast, voltage remains nearly constant at the LT8672 output during a cold crank event, and the downstream step-down regulator is able to maintain a 1.8 V output.
Numerous critical functions require regulated 5 V and 3.3 V rails, plus sub-2 V rails to power content, processor I/O, and core in analog and digital ICs. If VBATT drops below its outputs or VIN (MIN), a pure buck regulator would lose regulation if directly powered from VBATT. However, such critical functions typically do not require much power, so a highly integrated compact solution can be used, such as the 6 mm × 6 mm LT8603 quad output, triple monolithic buck converter plus boost controller.
The LT8603’s integrated boost controller works down to below 2 V, making it an ideal preregulator to its three buck regulators. Figure 10 shows a Power by Linear state-of-the-art solution for these applications that can ride through a cold crank event. The two high voltage buck regulators are powered by the preboost converter. When VBATT drops below 8.5 V, the boost controller starts switching and the output (OUT4) is regulated to 8 V. It can keep the output regulated with the input voltage down to 3 V once it is started. Therefore, the two high voltage bucks can ride through the cold crank condition, while providing constant 5 V and 3.3 V outputs, as shown in Figure 11. Once VBATT recovers to above 8.5 V from cold crank, the boost controller simply works as a diode pass through. The high voltage bucks can handle VBATT up to 42 V. The low voltage buck is powered from OUT2, providing 1.2 V through the cold crank event.
For always-on systems connected to VBATT for weeks or months without a battery recharge, light load and no-load efficiency are, in some cases, more important than full load efficiency. The Power by Linear family of ultralow quiescent current (IQ) devices preserve battery charge while withstanding challenging transient conditions and wide input voltage ranges, from 3 V to 42 V, and wide temperature ranges. To optimize efficiency and maintain regulation at light loads and no load, the regulator features Burst Mode® operation. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to a few microamps. In contrast, a typical buck regulator might draw hundreds of hundreds of microamps from VBATT when regulating with no load, draining the battery orders of magnitude faster.