Texas Instruments datasheet for the TPS51225C, a dual synchronous step-down controller with built-in 5 V and 3.3 V LDOs. Controllers of this family typically generate the always-on 3V and 5V system rails on notebook motherboards. The 1.51 MB PDF details the pin functions, enable and power-good logic, switching frequency selection and reference layouts. A technician can use it to check the VIN, EN and PGOOD pins and find out why the standby rails fail to start, before suspecting the switching MOSFETs or inductors.
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