It is the LT3751 High Voltage Capacitor Charge Controller, datasheet.

I am trying to calculate the required value for the input capacitor for my application, but the datasheet does not clearly specify a frequency.

The best clue I can find is the "One Shot Clock Period" in page 3, which at 44us results in 23kHz - but I am not entirely convinced.

  • \$\begingroup\$ Pretty sure that they don't write a number out because it's far more complicated than that - everything described on page 11-14 relates to the "frequency". \$\endgroup\$
    – pipe
    Commented Mar 7, 2019 at 7:33
  • \$\begingroup\$ yes exactly - the answer is sort of "up to 200kHz" - try searching in the datasheet for "khz" \$\endgroup\$
    – Taniwha
    Commented Mar 7, 2019 at 7:40

1 Answer 1


Edit (with corrections)

It looks like a high power hysteretic oscillator based on current limits and voltage feedback. So the flyback switching frequency will start low and then increase as it reaches the regulated output voltage which starts near 100kHz and up near 200 kHz.

Remember those camera flash chargers with the rising whistle sound? It's NOT Like that. It can charge a 1mF cap up to 500V in 1 second with the right parts. That's a lot of Joules (125). But it starts at near 100kHz and ramps up to 200kHz.

A forced reset allows things to settle every 26kHz period.

After a preset internal counter value at a regulated voltage, it shuts off. Below some voltage threshold with hysteresis, it starts up again.


Except as @pipe pointed out they reduced audio noise in this design, (except for a low load condition where it can cycle slowly in and out of no-load state.)

So they start the oscillator at 100kHz and use PWM to limit start peak currents then ramp up quickly to some R set current limit then increase in frequency as it charges up.

(snip) Internal control circuitry forces a one-shot condition at a periodic rate greater than 20kHz and out of the audio spectrum.

The regulation loop then determines the number of pulses that are required to maintain the correct output voltage.

Figure 5 shows the use of duty-cycle control. enter image description here

  • \$\begingroup\$ About the whistling: They do mention several times in the datasheet that they are avoiding the audio spectrum, so hopefully this switcher is more silent. \$\endgroup\$
    – pipe
    Commented Mar 7, 2019 at 8:34
  • \$\begingroup\$ kewl . @pipe you are a better man than I. You actually read the Theory of Operation. \$\endgroup\$ Commented Mar 7, 2019 at 8:40

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