[FAQ] TPS546D24A: What termination voltage can the PMBus / SMBus / I2C bus pins support and what pull-up resistors should be used?

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Part Number: TPS546D24A

Can the TPS546D24A PMBus / SMBus / I2C port be pulled up to 1.8V?

The TPS546D24A's digital interface pins PMB_CLK and PMB_DAT are designed to support SMBus 3.1 1MHz class High-Power digital interface. 

Per the SMBus 3.1 specification, their High/Low and Low/High thresholds are between 0.8V and 1.35V

This will support termination bus voltages between 1.62 (1.8V - 10%) and 5.25V (5 + 5%)

What pull-up resistance should be used with the TPS546D24A?

Per the high-power, 1MHz class  SMBus 3.1 standard, the TPS546D24A can support termination pull-up resistors with a current up to a maximum of 20mA when the CLK/DAT pins are pulled to 0.4V

For common bus voltages, the minimum termination resistance is:

1.8V    70-ohms

2.5V    105-ohms

3.3V    145-ohms

5.0V    230-ohms

Note:  The pull-up termination resistance is limited to the highest minimum termination resistance of any device connected to the bus.  If there are 400kHz class (6mA) or 100kHz class (4mA) devices sharing the same CLK and DAT nets, pull-up resistances will be limited to significantly higher resistors.

Depending on the speed, number of connected devices, and total net capacitance, the required pull-up current and maximum resistance will vary.  Each TPS546x24A device connected to the bus adds less than 5pF capacitance to CLK and DAT.  The pull-up resistors should be sized to at least meet the rise-time specification based on the CLK speed used.

1000ns for 100kHz

300ns for 400kHz

120ns for 1MHz 

Note:  The node voltage should rise to a minimum of 150mV above the minimum "High" threshold  (1.5V) and fall to 150mV below the maximum "Low" threshold (0.65V) within the specified rise time.

For a 1.8V bus voltage, reaching 1.5V (83%) will require approximately 2 time constants

For a 2.5V bus voltage, reaching 1.5V (60%) will require approximately 1 time constant

For 3.3V bus voltage, reaching 1.5V (45%) will require approximately 0.7 time constants

For 5V bus voltage, reaching 1.5V (30%) will require approximately 0.5 time constants

For a bus with 2x TPS546D24A devices plus an Microprocessor driving the I2C bus with 10pF pin (20pF total bus capacitance) with a 1.8V pull-up (2 time-constants rise time) operating at 1MHz (60ns maximum time-constant) a pull-up resistor less 3k-Ohms should be used.

Peter James Miller

Senior Applications Engineer

TI Power - Buck Switching Regulators - Mid Voltage

Senior Member Technical Staff 2020

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