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TL499A slow start-up, poor efficiency

Mike Farr
Prodigy 70 points
Other Parts Discussed in Thread: TL499A

Help please!  Anyone seen or have suggestion on this please?

A prototype 1.5V (AA alkaline cell) to 9V switcher circuit using TI guidelines doesn't work as intended.

1) It is very slow to start switching, around 5s to 10seconds.  Once the switching starts, it gets and hold output voltage perfectly under various load conditions.

2) It is inefficient, around 45% instead of the datasheet figure of 66%

Anyone seen these issues before, know of solutions?  Any help welcomed, thanks

  • 0
    TI__Guru**** 191445 points
    TL499A is a really old device, over 30 years since release. Can you post your schematic and some waveforms including the SW node during start up? I did not see any reference to efficiency estimates in the datasheet.
  • 0 in reply to JohnTucker
    Prodigy 70 points

    Many thanks for your kind response, John.

    Yes, we know it is an old device, but it is more recent than the TL496 it is replacing in a volume product of ours, and at least it is both RoHS compliant and available! This was our only reason to change.  But the change isn't as seamless as we hoped.

    I don't have a schematic easily to hand to post, but could do this later if necessary.  Even under no load, the slow start is always present, and the circuit is basically exactly as per application note, using a 47uH toroidal inductor - the same as works perfectly in our TL496 design.

    The switched node, pin 6, jumps to battery voltage initially, ie switch open, then it has a growing oscillation of aprox 900kHz which increases progressively over 5 or 10 seconds to about 1V p-p, before bursting into life with the expected waveform as per datasheet and working as intended, if low efficiency.

    You are correct, the TL499 datasheet doesn't specify efficiency, however TL496 gave 66ish% efficiency, so I assumed the TL499 replacement would be similar.  We are using it in the identical circuit as our existing TL496 based design, except for the Output Voltage setting divider which gives a very constant 8.74V output once it bursts into life.  I just can't see why a simple switcher which achieved 66% in the TL496 can only achieve 45% for the TL499A.

    Thank you for your time.

    Mike

  • 0 in reply to Mike Farr
    TI__Guru**** 191445 points
    If you can post your schematic, I can take a look. Also, I can see if I can locate someone who has used that part in the past.
  • 0 in reply to JohnTucker
    Prodigy 70 points

    John, here is the schematic:

  • 0 in reply to Mike Farr
    Prodigy 70 points
    Also, the TI Application Note - slva141.pdf - has a graph (Fig 7) clearly showing efficiency aprox 66% in the application we are testing. I knew I had seen the efficiency for this device published somewhere.
    Thanks for your help, John
  • 0 in reply to Mike Farr
    TI__Guru**** 164796 points
    Mike,

    Figure 7 is for 3V power input. Efficiency can be less for 1.5V
    What does battery voltage look like on oscilloscope?

    Does a 1.5V bench supply also power up slow?
  • 0 in reply to Ron Michallick
    Prodigy 70 points

    Thank you for your kind reply Ron.

    I appreciate Fig 7 is for 3V input and we could expect a lower efficiency, however I was expecting better than 40 - 45%!

    The circuit does this with both a 1.5V bench supply and a 1.5V  AA battery, all scoped to be a steady 1.5V.  This issue occurs for all input voltages, 1.0V - 1.7V of interest to us.  During the 'hanging' phase, the current draw is high, around 400 - 600mA.

    I found a blog which seems to be specifically commenting on this very problem: 

    http://www.piclist.com/techref/postbot.asp?by=time&id=piclist%5C2005%5C05%5C29%5C230455a&tgt=top

     ... and I quote:

     TL499 has a nasty high current mode
    when the power supply rises slowly - not mentioned in any data sheet
    or app note (of course).

    Reducing the input capacitor from 100uF to decrease any slow input rise-time definitely reduces susceptibility to this problem, though it doesn't improve the woeful efficiency. 

    As there appears to be an 'undocumented feature' which the blogger implies TI are not admitting to, we cannot take the risk replacing the trusty TL496 which has served us well for over 20 years and many 100,000s of products, with this TL499 in our product.  Unfortunately, because of this, we will be prototyping the LT1110 instead.

    Thanks again for you input.

    Mike