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Part 5 - Application Information

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The Enable Pin

The module features an enable pin that places the IC in shutdown meaning that the IC consumes, according to page 2 of the datasheet, less than 1μA. Of course, this quiescent current will be slightly higher in-circuit due to leakage currents on other connected pins; having said that, however, the module itself should have a similar quiescent current since it features no passive, resistive paths to ground, and all the other IC inputs would appear to go high-impedance during shutdown.

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Operating Voltage Range

The IC features and input range of 2.5V - 16V (when VBAT and VCC are tied together). The input voltage does not appear in the equation for calculating the output voltage or the equation for calculating the minimum transformer primary winding inductance. With this knowledge the only other possible limiting factors are:

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  • Transformer turns ratio;

  • Diode peak reverse-voltage rating;

  • Input capacitor voltage rating.

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By rearranging the equation for transformer turns ratio, and plugging in a turns ratio value of 8 to account for potentially using the FA2470 as opposed to the FA2469, the maximum input voltage for this limitation can be determined. This is shown below.

Next, the equation for the diode peak reverse-voltage can be rearranged in a similar manner:

Now, considering that this exceeds the voltage rating of the LT3420, this value can be disregarded and we can use the specified maximum of 16V for this limiting value.

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Finally, the input capacitor voltage rating can easily be selected based upon the application. Currently on my test module is a 10μF capacitor rated up to 35V (probably a bit overkill but never mind); however, this could be swapped for a lower voltage rating and/or higher capacitance to give improved bypassing for the module - this will help to reduce noise on the main power bus for the end circuit. Another viable alternative would be to replace the standard 1206 (3216 metric) footprint for that of a tantalum capacitor, especially if low input voltage are being used, anyway, I digress...

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The basic input voltage requirement of the module is a source between 2.5V and 15.5V although it would be wise to stick to maybe a 12V maximum because having headroom is always a luxury. Additionally, the minimum voltage should be taken with a pinch of salt, I have lit up an IN-19V Nixie tube from two, fresh, alkaline AA cells in series although the cathode illumination was beginning to get patchy.

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This would imply power sources from the following list:

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  • 1 - 3 Li-ion/Li-poly cells in series;

  • 1 - 8 alkaline or Ni-MH/Ni-Cd cells in series;

  • 6V or 12V sealed lead-acid battery;

  • 9V alkaline battery;

  • USB power bank/wall wart;

  • 9V/12V DC wall wart or rectified 9V AC wall wart;

  • Possibly a suitable number of supercapacitors in series.

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It should be noted that button/coin cells cannot be used due to their high internal resistance.

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Output Current Range

The output current of the module can range from about 2mA - 12mA with 100pF of refresh capacitance or from about 4mA - 14mA with 22pF of refresh capacitance (assuming a turns ratio of 16). The minimum load is required in order to prevent the output voltage from climbing to quite frankly ridiculous levels, I have measured (and felt) 540V on the output capacitor.

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