Part Number: TPS23881B
Other Parts Discussed in Thread: TPS23861, TPS23881, TPS23882
TPS23881B is an IEEE802.3bt compliant, 8-channel power sourcing equipment (PSE) that injects power onto CAT5 Ethernet cables. When using this device in any wired networking or industrial applications, it is important to implement the correct power-on timing of certain signals to ensure proper functionality of the device and prevent failure over time.
TPS23881B has two supply voltages: VPWR and VDD, nominally 54V and 3.3V respectively, powering the device's analog and digital circuitry. The device also utilizes a /RESET pin which resets the device, shutting off all ports and forcing registers to power-up state when asserted.
An IC's digital circuitry should always be controlled carefully, so that signals are interpreted correctly and the device never enters an undefined state of operation. Following the power-on sequence of the VPWR, VDD, and /RESET pins is vital to controlling the digital state of TPS23881B. As specified in the device's datasheet, the /RESET pin must be held low until VPWR and VDD have reached or are above their undervoltage-lockout (UVLO) thresholds.
Maximum UVLO rising thresholds of VPWR and VDD are 18.5V and 2.75V respectively. The /RESET pin should be held low or under 0.9V until the two supply voltage pins have hit these UVLO thresholds. This can be done using an RC network delay between the /RESET and VDD pins or by having an external MCU control the rise of this pin.
The power-on sequence can be seen in the following diagram:
Shown below is what the rising behavior of these three signals should look like. These waveforms have been captured using the TPS23881B evaluation modules.
The contents of this FAQ also apply to other power sourcing equipment in TI's Power over Ethernet portfolio, such as the TPS23881, TPS23882/2B, TPS23861. UVLO thresholds of VPWR and VDD might very depending on the device - these values can be found in the Electrical Characteristics of each device's datasheet.
Additional resources to refer to:
TPS23861 Power-On Considerations