Device description

L6984

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DocID025378 Rev 3

A standard COT loop requires a high ESR output capacitor to generate a proper PWM

signal. The L6984 device supports output ceramic capacitors with negligible ESR.

below the valley voltage threshold. The L6984 on-time is calculated as:

Equation 6

the overall contribution given by the integrated capacitor and the parasitic capacitor of the

board trace at the pin 3.

depends on the parasitic capacitance connected at the pin 3 (TON) that changes over the

different board layouts. If the final application requires precise TON adjustment, a further

measurement.

The L6984 internal circuitry compensates the TOFF time variation over the input voltage

range to keep the switching frequency almost constant.

The almost constant switching frequency depends on the output current and it can be

calculated as:

Equation 7

Equation 8

Table 5 on page 7) and DCR the equivalent series resistor of the selected inductor.

The L6984 small signal loop compensates the internal losses, that depends on the output

current value, adjusting the TOFF time so the switching frequency.

Finally from Equation 6 and Equation 7:

Equation 9

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=

=





----------------------------------

=



+

 I

OUT



+

–

 I

OUT



+

------------------------------------------------------------------------------------

=

1

0.9

--------

*







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

=