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Original question:

SN6505B: SN6505B application

SN6505B: SN6505B Frequency Deviation

Uros Platise
Prodigy 135 points
Part Number: SN6505B


Hi,

The SN6505 data-sheet states:

  • average switching Frequency (SN6505B), is from 363 to 517 kHz, typ 424 kHz

Figure 31 then shows pretty stable frequency operation within around 420 .. 435 kHz and wide Vcc range 2.25 V to 5.5 V of better than +/-2% stability.

Therefore I wonder:

  1. under what circumstances the worst specifications like 363 and 517 kHz applies?
  2. for devices and circumstances for which this worst case applies, would this refer to initial accuracy offset, or temperature or long-term stability?

Our application: we are syncing to sn6505 at the output and synchronously drive mosfets to reduce losses of the rectifier, and would like to understand and separate initial frequency deviations from possible short-term and long-term drift to understand and properly handle all of the (expected) worst cases.

Kind regards,
Uros

  • 0
    TI__Genius 17100 points

    Hi Uros,

    Thank you for posting to E2E! For SN6505B transformer compatibility calculations of V-t product, we recommend always using the minimum switching frequency specification of 363kHz.

    Alongside this, the datasheet range covers SN6505 devices' switching frequency variation from operating conditions and manufacturing. Switching frequency is pretty consistent across operating Vcc levels, and it decreases at lower temperatures and increases as temperature increases, however this is not applicable when external clocks are connected to the SN6505's CLK pin as the device will follow the CLK input instead of using its internal oscillator circuit.

    Will you be syncing multiple SN6505 units with an external clock or just one?


    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 135 points

    Hi Manuel,

    in our case I use internal freq generation, and not the external, due to the SSM and the hardware logic (timers) sync to the output transitions and then (after) start to drive MOSFETs to improve efficiency. So therefore I am interested in internal clock generation, when should I expect to face the worst case limits from the data-sheet?

    Or with other words, after assembly I notice frequency is around 415 kHz, some 418 kHz, ... for this initial freq will maximum deviation be as in Figure 31, however in our case input Vcc is always 5 V +/- 1%.

    Kind regards,
    Uros

  • 0 in reply to Uros Platise
    TI__Genius 17100 points

    Hi Uros,

    I understand, thank you. Worst-case limits from the datasheet encompass best-case and worst-case variations in temperature, supply voltage (Vcc) levels, and most importantly in this case, manufacturing processes.

    The biggest differences in this spec will be due to process variation from device to device. For each device, we don't expect frequency to range much at different Vcc levels or temperatures, as shown by Figure 31 in the datasheet and confirmed by your measurements.


    Thank you,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 135 points

    Thank you Manuel.