0.55 W, 88%, 78 kHz, 48 V-to-5 V Fibonacci Hybrid DC-DC Converter IC Using 66 mm³ of Passive Components With Automatic Change of Converter Topology and Duty Ratio for Cold-Crank Transient

For 48 V mild hybrid vehicles, a sub-0.5 W, 48 V-to-5 V dc-dc converter fulfilling: 1) high efficiency with small volume; 2) constant switching frequency of less than 500 kHz to avoid the frequency band of AM radio; and 3) constant output voltage (V-OUT) against the sudden drop of input battery voltage (V-IN) under cold cranking is required. To meet the requirements, a 0.55 W, 88%, 78 kHz, 48 V-to-5 V Fibonacci hybrid (FH) dc-dc converter IC using 66 mm(3) of passive components is proposed. In the FH dc-dc converter, by adding an inductor and an output capacitor to a 1/5 Fibonacci switched-capacitor (SC) dc-dc converter, the SC dc-dc converter also works as a buck converter without adding power transistors. When V-IN drops from 48 V to 20 V in 1 ms in the automotive cold cranking, the FH dc-dc converter cannot keep 5-V V-OUT, because the output voltage of the internal 1/5 SC dc-dc converter drops from 9.6 to 4 V and the internal buck converter cannot generate 5-V V-OUT from 4 V. To solve the problem, a new control method named automatic change of converter topology and duty ratio (ACCD) is proposed. In ACCD, when V-IN drops less than 31 V, the converter topology automatically changes from 1/5 SC dc-dc converter to 1/3 SC dc-dc converter and the duty ratio of the pulsewidth modulation signal automatically decreases 3/5 times, thereby achieving the constant 5-V V-OUT under cold cranking. To reduce the volume of the FH dc-dc converter, all transistors and diodes, including ten power transistors, gate drivers, bootstrap circuits, and the controller, are fully integrated on 4.6 mm x 2.3 mm IC fabricated with 180 nm BCD process. In the measurements, the proposed 0.55 W, 48 V-to-5 V FH dc-dc converter IC achieved the highest efficiency (88%) with the smallest volume of passive components (66 mm(3)) at the lowest switching frequency (78 kHz) compared with previous publications.