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LM2621: Simplest SEPIC schematic for low VIN (1.2 - 4.5) with VOUT 3.3

Jason Carnahan
Prodigy 80 points
Part Number: LM2621
Other Parts Discussed in Thread: TPS63802

I am trying to design the simplest schematic for the LM2621 in a SEPIC configuration to run off of 3x 1.5v batteries.  I need a fixed 3.3v out.  The batteries will start at 4.5v and I want to run as low as possible (down to 1.2v hopefully).  It seems the datasheet SEPIC design can only run down to 2.5v and I need to run with a lower VIN than that.  I have seen other schematics from other sources but would like to have a recommended solution from a TI expert with the simplest schematic possible (other solutions accommodate higher VIN or VOUT which I believe complicates the design).  Is there a known/recommended design that matches these parameters? (VIN=1.2v-4.5v, VOUT=3.3v).  Thanks.

  • 0
    TI__Mastermind 34221 points

    Hi Jason,

    Thank you for reaching out.

    May I know the maximum load current? Have you ever considered the four-switch buck-boost converter? 

  • 0 in reply to Zack Liu
    Prodigy 80 points

    Hi Zack,

    Max current load is 500mA.  I have looked at several of the buck-boost options.  I may try to use them in the future but am seeing availability issues in the short term so I will continue to use the LM2621 if possible since I use it in other boost-only designs and have access to a supply source.  Any info on the SEPIC (buck-boost functionality) would be useful.  Thanks.

  • 0 in reply to Jason Carnahan
    TI__Mastermind 34221 points

    Hi Jason,

    TPS63802 may meet your requirement. Its minimum startup voltage is 1.8V and minimum working Vin is down to 1.2V. 

    If you still would like to use LM2621, please refer to datasheet figure 18. The VDD, BOOT, FREQ, ONOFF pin can be connected to Vout so that the minimum Vin is not 2.5V. Please note that the peak current through the main switch is IIN+IOUT+0.5xDeltaIL1+0.5xDeltaIL2, when Vin is low, it may trigger the switch current limit.

  • 0 in reply to Zack Liu
    Prodigy 80 points

    I will look into the TPS63802, I do find one supplier that might have sufficient stock. 

    I do want to finalize my LM2621 design if possible.  I have tried VDD,FREQ,ONOFF connected to VOUT (through a 390 Ohm resistor) but it did not work unless I switched them to connect to VIN instead (but only down to 2.5v, as discussed).  I have not tried switching BOOT to connect to VOUT (it is connected to VIN).  Can you show me a sample schematic of how I would connect everything?

  • 0 in reply to Jason Carnahan
    TI__Mastermind 34221 points

    Hi Jason,

    According to LM2621 datasheet page 9 part 7.3.2 Low Voltage Start-up: The LM2621 can start-up from input voltage as low as 1.1V. On start-up, the control circuitry switches the N-channel MOSFET continuously at 70% duty cycle until the output voltage reaches 2.5V. After this output voltage is reached, the normal step-up regulator feedback and gated oscillator control scheme take over.

    The above description refers to the Boost converter topology. As LM2621 is a non-synchronous boost converter, once the Vin is connected to externa power supply, the Vin will charge the output capacitor to Vin-diode voltage drop. So BOOT, EN, VDD voltage is Vin- diode voltage drop.

    If you configure the device in a SEPIC topology and connect VDD, EN, BOOT pin to VOUT, as the output voltage is always 0V, so the device doesn't switch as you found. But if connect VDD, EN, BOOT pins to VIN, this device cannot work below 2.5V.