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TPS25751: VIN_3V3 Pin Connection for USB-C Charging Design

Ido David
Prodigy 20 points
Part Number: TPS25751

Tool/software:

Hello everyone,

I'm using the TPS25751D in my design to charge a 6-cell Li-ion battery pack via USB-C, with fast charging at 20V and 5A. I have a few questions about the setup and would appreciate any insights.

In my design:

  • I've connected all VBUS and VBUS_IN inputs together and left the PP5V pins unconnected.

My question is about the VIN_3V3 (pin 36). Since the TPS25751D has a built-in 3.3V LDO, I’m not sure if the VIN_3V3 input needs a specific connection, or if it can be left floating. Additionally, with only USB-C as the power source, what would be the recommended source for 3.3V?

Any advice or recommendations would be greatly appreciated. Thank you!

  • 0
    TI__Mastermind 20057 points

    Hi Ido, 

    Thank you for reaching out!

    • Please connect PP5V to GND if unused (this would only be valid in sink-only applications)
    • See the following battery charging + PD reference design, in which VIN3V3 is generated from an LDO 

    Please let me know if you have additional questions/concerns. 

    Best Regards, 

    Aya Khedr

  • 0 in reply to Aya Khedr
    Prodigy 20 points

    Hi Aya,

    Thank you for your response!

    I'm a bit confused about this component because it supports bidirectional operation. However, I intend to use it solely for charging a lithium battery pack via a Type-C connection. Based on the datasheet, I understood that PP5V functions as an input, so I left it floating. However, from your comment, it seems that PP5V is a 5V output, which can also serve as an input to an LDO.

    Could you clarify if I need to connect the LDO's output back to the component (Vin_3.3)? Does this mean I should utilize PP5V in my application? If my understanding of your explanation is incorrect, could you please clarify how Vin_3.3 and PP5V should be used in this context?

    Additionally, the component has an internal LDO that provides 3.3V. Why is an external LDO included in the schematic?

    Looking forward to your clarification.

    Best regards,

  • 0 in reply to Ido David
    TI__Mastermind 20057 points

    Hi Ido, 

    1- Your understanding is correct, PP5V is a 5-V input system supply to VBUS. This is the internal 5V/3A power path that can be utilized for sourcing. Hence, PP5V would not be used in a sink-only application and should be connected to GND. 

    2- The TPS25751 is powered from either VIN3V3 or VBUS. The normal power supply input is VIN3V3. When VIN3V3 power is unavailable, and power is available on VBUS, it is referred to as the dead-battery start-up condition. When powering from a VBUS input, the voltage on VBUS is stepped down through an LDO to LDO_3V3.(see below)

    I noticed that the ADCINx pins are floating in the schematic above. The ADCINx inputs to the internal ADC control the behavior of the TPS25751 in response to VBUS being supplied when VIN_3V3 is low (that is the dead-battery scenario). These pins must be tied to LDO3V3 via a resistive divider. See section 8.3.6 in the datasheet for more information. 

    Also, see the following FAQ for sink-only designs: e2e.ti.com/.../faq-tps25750-how-do-i-create-a-sink-only-usb-c-pd-port-to-replace-a-legacy-barrel-jack-connector

    Please let me know if you have additional questions. 

    Best Regards, 

    Aya Khedr 

  • 0 in reply to Aya Khedr
    Prodigy 20 points

    Thanks, Aya!

    From what I understand, I can tie this pin to ground and operate permanently in "dead-battery" mode.

    However, I’m uncertain about the correct configuration for ADCINx.

    I checked the link you shared and noticed that the resistors R1, R3 (DNI), and R2, R4 are set to 0 Ω. Does this configuration work exclusively in SafeMode?

    My design closely resembles the one in the link. It operates in sink-only mode, with a blower as the load. I want to charge my batteries (12-cell Li-ion pack, 6s2p) as quickly as the source can supply (20V, 5A) but also allow charging at a lower power level (5V) if that’s the only supply available. I don’t use an Optional Embedded Controller, but I do include an EEPROM.

    I’m trying to determine the best Dead Battery Configuration for my design and requirements. Specifically:

    • For example, what is the difference between the first four lines in the configuration table? They all use AlwaysEnableSink. How should I decide which line fits my design?
    • How do I choose the resistor values for this configuration?

    Additionally, as I mentioned, I don’t have an Optional Embedded Controller. How can I configure the ADCINx inputs? Does the device just load a configuration from an external EEPROM?

    Any guidance or recommendations would be greatly appreciated!

    Best regards,

    Ido

  • 0 in reply to Ido David
    TI__Mastermind 20057 points

    Hi Ido, 

    Ido David said:

    From what I understand, I can tie this pin to ground and operate permanently in "dead-battery" mode.

    Your understanding here is correct. 

    Ido David said:
    I checked the link you shared and noticed that the resistors R1, R3 (DNI), and R2, R4 are set to 0 Ω. Does this configuration work exclusively in SafeMode?

    SafeMode is the typical dead-battery setting used when loading the configuration from the EEPROM. This would be the case in the sink-only example shared. I believe this would be best suited for your design as well.

    AlwaysEnableSink is only needed if you need to power the system to configure the PD controller (i.e. when using an EC to load the configuration). 

    In terms of the I2C Address Index setting, this would set the PD controller's target address (there are 4 options, it is up to you). The I2Ct lines can optionally be connected to an external MCU to read status registers, therefore an address would be needed here. 

    See below table for resistance values and how they correlate to the ADCIN decoded values:

    Please let me know if you have additional questions/concerns.

    Best Regards, 

    Aya Khedr 

  • 0 in reply to Aya Khedr
    Prodigy 20 points

    Thanks, Aya!

    Aya Khedr said:
    The I2Ct lines can optionally be connected to an external MCU to read status registers, therefore an address would be needed here. 

    I just want to clarify whether I can use the ADCINx pins even though my design doesn’t include an optional embedded controller. My setup only consists of a PD controller, Charger IC, and EEPROM.

    Referring to the documentation:
    8.4 Device Functional Modes
    8.4.1 Pin Strapping to Configure Default Behavior
    During the boot procedure, the device reads the ADCINx pins and sets the configurations based on the table below. The device then attempts to load a configuration from an external EEPROM on the I2Cc bus. If no EEPROM is detected, the device waits for an external host to load a configuration.

    From this, it seems like I can use the ADCINx pins, since the device first attempts to load a configuration from the external EEPROM on the I2Cc bus, which is part of my design. Could you confirm if this understanding is correct?

    Best regards,

    Ido

  • +1 in reply to Ido David
    TI__Mastermind 20057 points

    Hi Ido, 

    Yes, the ADCINx pins are used to set the dead-battery configuration which in your case, should be set to SafeMode. These pins can be tied to GND for a decoded value of 0 (see table 8.5 in the data sheet for reference) 

    Best Regards, 

    Aya Khedr