The S19 hydro hash board consists of 104 BM1398BB chips (silk screen sequence BM1-BM104) divided into 52 groups (domains), each group consisting of 2 ICs. The BM1398 chip used in the S19 Hydro hash board operates at 0.37V. In groups 52, 51, 50, 49, 48, 47, and 46 (7 groups in total), the 25V output by the boost circuit U9 supplies power to the LDO and outputs 1.8V. Group 45 to Group 1 are provided 1.8V by VDD 18V through LDO. Each time one domain is moved back, the domain voltage decreases by 0.37V. All 0.8V is provided by the 1.8V of this domain through the LDO output.
Source: Antminer S19 Hydro Hash Board Repair Guide | Zeus Mining