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LM5088: What value to use for Vin(max) in and automotive application?

John Whitmore
Intellectual 270 points
Part Number: LM5088


I'm going through the datasheet for the LM5088 trying to estimate values for the various parts required. The formula in Section 9.4.2 to calculate the Inductor value uses Vin(max) and of course I'm confused as to what value I'd use for that. On the face of it it's a 12V automotive system but when the engine is on and charging I'd expect the Voltage of the system to be about 14.5Volts. Lets say 15V to keep with nice round numbers. So I've got 15V as the maximum input voltage.

But hold on a minute the only reason I'm here is I'm trying to power a 5V uC board in an environment which will potentially have load dumps forcing the input voltage to spike up to 100V perhaps even 150V so is that the maximum voltage I should be putting into the formula.

Actually am I even looking at the right part here? Is there a better DC/DC converter which will cope with automotive load dump and inductive loads

  • 0
    TI__Genius 11040 points
    Hi John,

    I recommend using the maximum input voltage that the LM5088 will see. As a starting point I recommend using the Quickstart calculator in the product folder which will help calculate the components in the applications circuit.

    For 12V battery systems the unsuppressed load dump conditions are specified at 65-87V, and for 24V battery systems the unsuppressed load dump conditions are specified at 123-174V. However, typically car manufacturers design the alternator with a centralized Transient Voltage Suppressor (TVS) to protect against the load dump conditions. This leads to most manufacturers defining the level at which they will be protecting the downstream components. Typically, a 36-60V wide input converter or controller is used for a 12V battery system, and this regulator is paired with a TVS to protect against the manufacturer's defined load dump level. Typically, a 60-100V wide input converter or controller is used for a 24V battery system, and this regulator is paired with a TVS to protect against the manufacturer's defined load dump level.

    Please let me know if you have any additional questions.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Intellectual 270 points

    Thank you Katelyn, I should have mentioned that I intend putting in a TVS Diode to filter out spikes above 75V, or there about, so I'll use 75V as my maximum input voltage. I can't rely on manufacturer's centralised TVS protection for load dump as I'm retro fitting to older vehicles and not sure I can trust any such specs.

    The Quickstart is great but it's using all polarised Caps, which I try and minimise the use of. Ceramics rock ;-)

    Thanks again for responding to my question and I'll hopefully get a circuit that sort of works, I'm embedded SW so should not be let loose with a soldering iron.

  • 0 in reply to John Whitmore
    Intellectual 270 points
    Actually the more I look at the datasheet and the quickstart it looks like the default Capacitor is a polarised and it's just used everywhere without any thought going into it. Do all 11 Caps have to be polarised? Tin cans everywhere.
  • 0 in reply to John Whitmore
    Intellectual 270 points
    Sorry... back to the polarized Caps there are two such caps placed back to back on the COMP pin in the quick start spreadsheet. To be fair the same configuration of two polarised caps is included in the datasheet. However the quickstart design I have created gives a value of 668pF so that's not an aluminium cap. Are these tantalum caps? The datasheet doesn't mention anything about these caps. Well if it does I missed it. I'll have to go back over it again.
  • 0 in reply to John Whitmore
    TI__Genius 11040 points
    Hi John,

    Can you share your quickstart spreadsheet in an attachment? Then, I can review the results and provide feedback.

    Thank you,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Intellectual 270 points

    That would be brilliant Katelyn, if you had time for it. I do think that there's a few discrepancies between the Datasheet and the quickstart. Actually the datasheet contradicts itself in places. Section 9.4.6 Input Capacitors, for example says in one paragraph "Quality ceramic capacitors with a low ESR should be selected for the input filter." and then in the next paragraph of that section "Because of those conditions, it is recommended that either an aluminum type capacitor with an ESR or increasing CIN>10 x LIN While using aluminum type capacitor care should be taken to not exceed its maximum ripple current rating"

    Actually then the quickstart has a single Aluminium whereas the datasheet ends up using 5 ceramics 2.2uF

    I'll attach what a SW Engineer has come up with ;) All I can say is automotive is a pain!

    If you did have time to cast your eye over it I'd owe you a tasty beverage of some description.

    Not sure my attachment is working out, might have to try a second time

    2112.LM5088_Quick_Start.xls

  • 0 in reply to John Whitmore
    Intellectual 270 points

    Another question which I'm left with, because it's not dealt with in the Datasheet is the thermal relief pad under the component. It's not mentioned in the Pin list at the start of the document and later on in section 10 it suggests thermal relief vias be placed under the component but the document says nothing about it's electrical connection. Can I assume it can be connected to the ground plane of a design? Perhaps it's meant to be connected to the input voltage?

    I'll have to go through the document again it seems a very strange omission from the datasheet to forget one its connections.

  • 0 in reply to John Whitmore
    TI__Genius 11040 points
    Hello John,

    I apologize for the delayed response on your questions.

    First of all, the thermal pad should be connected to ground. You can see in the Thermal Considerations section of the datasheet that the schematic shows the pin "EP" connected to the ground. You can also see an example of this connection in the EVM design:

    www.ti.com/.../snva379e.pdf

    Second, looking at your Quickstart file I understand the confusion around the polarized capacitors. This same symbol is used for all capacitors in the schematic in the QuickStart file. Ceramics or low ESR aluminum/polarized capacitors can be used for all capacitors.

    For example, CIN is calculated to 6uF in the Quickstart file. This is the minimum input capacitance required. This can be a combination of ceramics, ceramics + polarized capacitors, or polarized capacitors. A second example, COUT can be a combination again with a maximum ESR of 103mOhm.

    Please let me know if you have any additional questions.

    Best Regards,
    Katelyn Wigenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Intellectual 270 points

    Thank you Katelyn that answers all my questions. Hopefully I can get this onto a PCB Design and try it out.

    John