7.5.49 VREF_TRIM (MFR_SPECIFIC_04) (D4h)

The VREF_TRIM command applies a fixed offset voltage to the Error Amplifier reference (EA_REF) voltage. It is most typically used to trim the output voltage at the time the PMBus device is assembled into the end user’s system. The contents of this register can be stored to nonvolatile memory using the STORE_DEFAULT_ALL command.

For loop slave device, this command cannot be accessed. Any writes to this command will be ignored. An attempt to read or write this command will result in a NACK’d command, the reporting of an IVC fault, and triggering of SMB_ALERT.

The settings of the VOUT_MODE command determine the effect of VREF_TRIM command. In this device, the VOUT_MODE is fixed to Linear with an exponent of –9 (decimal).

The maximum trim ranges between –64*1.953 mV to +63*1.953 mV in 1.953-mV steps.

If a value outside this range is given with this command, the device sets the reference voltage to the upper or lower limit depending on the direction of the setting, asserts SMBALERT and sets the CML bit in STATUS_BYTE and the invalid data bit in STATUS_CML.

The value of EA_REF including VREF_TRIM is also limited by the values of VOUT_MAX, VOUT_MIN, VOUT_COMMAND, VOUT_SCALE_LOOP and STEP_VREF_MARGIN_HIGH/LOW. See VOUT_MAX and VOUT_MIN for additional details.

The EA_REF voltage transition occurs at the rate determined by the current state:

• Soft-Start: TON_RISE command
• Steady-State: VOUT_TRANSITION_RATE command
• TOFF_DELAY: VOUT_TRANSITION_RATE command
• Soft-Stop: TOFF_FALL command

The VREF_TRIM has two data bytes formatted as two’s complement binary integer and can have positive and negative values.

High Byte:

default: 0000 0000 (binary) 0 (decimal)

Minimum: 1111 1111 (binary) (sign extended)

Maximum: 0000 0000 (binary) (sign extended)

Low Byte:

default: 0000 0000 (binary) 0 (decimal)

Minimum: 1100 0000 (binary) –64 (decimal) (–125 mV) (sign extended, two's compliment)

Maximum: 0011 1111 (binary) 63 (decimal) (123 mV)