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LMZM33602: Correct and full negative output implementation (IBB)

Ofer Saferman
Prodigy 70 points
Part Number: LMZM33602
Other Parts Discussed in Thread: LMZM33603

Hello,

I have a design with Vin=28-30V and Vout=-6V at 1A.

I have followed application note SNVA800B regarding an inverting buck-boost application of the LMZM33602.

I think it is missing some information.

SNVA800B shows RT pin floating and states that all data is for 400 KHz, yet when I implement the design using Webench (Using the product page for LMZM33603 because only it offers the possibility to design IBB) the recommended frequency is 760 KHz but the RT pin is still floating. Shouldn't an RT resistors be connected to modify the frequency?

How should the RT resistor be connected if a different frequency than 400 KHz is required? Should it still be connected to AGND (which is now -Vout)?

I need to use the SYNC function but I don't need the enable function.

How would the SYNC pin connection differs from the regular Buck connection?

Is it still a pull-up resistor on the SYNC/EN pin to Vin and a series 1nF capacitor to feed the SYNC signal?

Is there any additional level-shifting required or any protection on the SYNC pin (driven by an FPGA) so that negative voltage spikes will not appear on it and harm the FPGA pin?

Thanks.

  • 0
    TI__Guru 50120 points

    Hi Ofer,

    I will contact the Webench team to get this fixed. If Rt pin is left open and floating, the device will default to 400kHz as detailed in the datasheet. 

    If you want to configure the switching frequency you will still have to connect the Rt resistor to the IC return which in this case is -Vout.

    As for external synchronization, you would need additional level shifting circuit. Have a look at section 7.2 in the app note below for a simple implementation of this circuit.

    6644.Working with inverting buck-boost converters (1).pdf

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 70 points

    Hello Jimmy,

    Thank you for the prompt response.

    Regarding the application note for IBB (SNVA856) that you directed me to and the level shifting circuit for SYNC -

    It seems that in that application note the SYNC level shifters are intended for DC-DC converters which employ a direct connection of the SYNC pin which is referenced to ground. This is not the case for the LMZM33602/3 which already requires an AC-coupled SYNC signal even for a regular buck application without the DC restorer resistor and diode appearing in SNVA856. From this I gather that the LMZM33602/3 probably has an internal DC bias circuit on the SYNC pin. Thus, I am wondering if the level shifter for SYNC is still required for this part as well?

    Since the SYNC pin for the LMZM33602 is also the enable pin it will still require the pull-up resistor to Vin, otherwise the part will be disabled. I don't think that it will co-exist with the DC restorer circuit appearing in SNVA856 for SYNC.

    Thanks,

    Ofer.

  • 0 in reply to Ofer Saferman
    TI__Guru 50120 points

    Hi Ofer,

    I see what you mean. Figure 21 in datasheet matches the lower schematic solution in that app note. In the case, there may not be a need to level-shift as long as you are controlling the switching frequency with a low impedance signal as mentioned in the app note. Since the module combines the features of EN  and SYNC into one pin, so long as you pull EN up to VIN, the SYNC pin will never be floating and you will not need that lower 200kOhm resistor. However I'd advise to have that diode as a safety protection during startup. 

    I'd also recommend taking a LMZM33602/3 EVM, configure it for IBB and testing to check desirable operation.

    Regarsd,

    Jimmy