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TPS61240-Q1 Output Current Capability

Other Parts Discussed in Thread: TPS61240-Q1, TPS61029-Q1

Hi Team,

I am evaluating the TPS61240-Q1 for the following specs:

Input: 3.3V

Output: 5V @ 600mA

Now, when I look in the datasheet, at 3.3V input, it looks as though this part will not work for 600mA output:

However, when I run a webench design, it looks as though this part can run at those specs:

I am inclined to believe that the webench simulation is actually more accurate since my input voltage is not that low as compared to the output voltage, but will need evidence that the part can handle the 600mA output current.

Please let me know if the webench is actually reflecting the correct value of output current and please let me know where in the datasheet I should be looking to fully understand output current capabilities.

Thank you,

Jared

  • Jared,

    The appropriate applications engineer has been notified of your post and will respond accordingly.

    Regards,
  • Jared,

    I suggest you look into a heftier part, like the TPS61085-Q1, and set the output voltage to 5V with the traditional resistor divider into the feedback (FB) pin.

    Although the fixed output voltage of the TPS61240-Q1 may be tempting, I am quite sure that the TPS61240-Q1 DOES NOT support 600-mA output current. Allow me to explain how I came to this conclusion: 

    From one of the first lines of the datasheet, "the TPS6124x supports output currents up to 450mA."


    I think the graph you are referring to is a good depiction of the performance of the part, and this data should be trusted over WEBENCH.

    The statement you made: "my input voltage is not that low as compared to the output voltage" is confusing me, since I can plot your conditions on the Figure 1 graph and determine that with VIN=3.3V, VOUT=5V (constant) the maximum output current is ~360mA

    The best graph to look at might be Figure 5, which shows the maximum output current in a very dramatic fashion:

    Here you can see that no matter what the input voltage, the output current is going to drop off a cliff before 500mA. At VIN=3.3V, the maximum output current is still very clearly between 300-400mA before VOUT<5V.

    The TPS61240-Q1 internally limits the output current by monitoring the peak "valley current" through the internal switch. Since the switch itself is limited to 600mA typical, and output current equal to or greater than this value cannot be achieved. I'm still not an expert on calculating the true maximum output current of a boost based on the maximum switch current, but I would suggest looking for ILIM or ISW,LIM (min) > 2*IOUT. This estimate will vary a lot based on efficiency (duty-cycle), delta-IL, and VOUT-VIN, but it may prevent you from getting your hopes up too soon on a part that won't fit the needs of an application.

  • Hello Brian,

    Thank you for the explanation.

    If you market the TPS61240-Q1 as a device that can only output 450mA of current, then you should consider talking to the webench team so the webench design is reflective of the datasheet.This is very misleading to customer who knows that the cover of a datasheet is generally worthless from a specification perspective and is seeking to evaluate their design by running a webench simulation, which should ultimately match what is in the datasheet.

    I am considering the TPS61029-Q1 instead, which can handle a switch current of up to 1800mA.

    Would you also recommend this as a device to fit the specifications above?

    In the TPS61029-Q1 datasheet, it says there is a 5V fixed version, called the TPS61027-Q1:

    However, when I go to order the TPS61027-Q1 (automotive grade) there is no orderable part:

    Can you please direct me to the orderable part for the TPS61027-Q1?

    Thank you,

    Jared

  • Also, I took a look at the TPS61085-Q1. Having a switcher that has a "selectable frequency of 650 kHz or 1.2 MHz" falls directly in the AM band:

    Thus, this device would likely be disqualified from any discussions with automotive customers.

    Thank you,

    Jared

  • Yes, the minimum frequency of the TPS61085-Q1 is listed as 480kHz, but I see that it is the minimum value when the typical value is 650kHz.

    The TPS61029-Q1 looks like it will work in your application. The TPS61027-Q1 says "Product preview" in a table note, so it has not been released and there is no orderable part number for this device.

    What is wrong with using the TPS61029-Q1 and a resistor divider feedback (FB) network for setting the output voltage at 5V?

    If the Fixed-Voltage feature is a must-have, we will need to take this conversation offline to discuss.

  • Brian,

    "the minimum frequency of the TPS61085-Q1 is listed as 480kHz, but I see that it is the minimum value when the typical value is 650kHz."

    Yes, you are correct, which means you cannot guarantee a frequency of 480kHz. The minimum frequency of 480kHz would be spec'd in this case for a worst-case evaluation by the customer, not so that a customer can set their switcher to switch at that frequency. Thus, as I mentioned, this part would be disqualified from automotive application under most circumstances (including this one).

    "What is wrong with using the TPS61029-Q1 and a resistor divider feedback (FB) network for setting the output voltage at 5V?"

    Using a resistor divider to provide feedback will require more quiescent current. Typically, an automotive module is specified to allow only 100uA of quiescent current in "sleep" mode. In "sleep" mode, the micro will still need to have power along with the CAN transceiver, thus the converter supplying the micro and CAN's power factors into the "sleep" mode budget. In the case of the TPS61029-Q1, the difference in Iq can move between 25uA and 45uA:

    This difference of 20uA has a huge impact as to whether or not the part can be used. In order to minimize the Iq, fixed output parts are typically used.

    Let's talk offline about the TPS61027-Q1 and thank you for your support.

    Best regards,

    Jared