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LM2662 Oscillation Frequency

Michael Brantley
Intellectual 1535 points
Other Parts Discussed in Thread: LM2662

I have an LM2662 used as an inverter with an input voltage of 5V and an output voltage of -5V. The Fc pin is connected to Vin. During startup, does switching start at 150Khz or is there some 20kHz switching till Vin completes the ramp up?

The model we have developed so far has the part switching at 20Khz until Vin reaches 4V. Is that the correct way to model during Vin ramp up?









  • 0
    TI__Expert 4565 points

    Hi,

    I haven't used this device before, but looking at the datasheet, OSC freq seems already reaches near its final value even with lowest Vin level(1.5V) assuming FC is connected to V+. So I think you can model the starting point of high freq much earlier than 4V(like lower than 1.5V which is not normal operation range anyway).(page 5 of attached datasheet)

    And the OSC freq is 20kHz or 150kHz, but actual switching freq is 1/2 of OSC freq.(page 3 of attached datasheet)

    Hope this helps,

    lm2662.pdf
  • 0
    TI__Prodigy 140 points

    The device should be modeled according to "Figure 12" on "page 5" of the LM2662/63 datasheet.  Depending on what your Vin ramp looks like, the device will have a typical oscillator frequency that rises with the rising Vin.  150kHz is specified at V+ = 5V, FC = 5V, LV = GND, and OSC = open.  Supply voltages below 5V (for example during a Vin ramp) will result in oscillator frequencies below 150kHz, as shown on Figure 12 on page 5.

    Please keep in mind, the switching frequency of the flying capacitor is 1/2 the oscillator frequency.  Therefore, the swtiching frequency is 75kHz when the oscillator frequency is 150kHz., and the switching frequency is 45kHz when the oscillator frequency is 90kHz.

  • 0
    TI__Intellectual 2140 points

    Please see Karl's response.