L6984 datasheet(14/34 Pages) STMICROELECTRONICS

14 / 34 page

Device description

L6984

14/34

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.

The controller generates a TON duration switching pulse as soon as the voltage ripple drops

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

Equation 6

where RTON represents the external resistor connected between VIN, TON pins and CTON

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

board trace at the pin 3.

CTON value for the L6984 soldered on the STM evaluation board is 7.5 pF typical, anyway it

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

fine tune for RTON value may be required accordingly with the designed board layout. This is

simply done adjusting the RTON value to get the desired TON value with direct scope

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

where DREAL is the real duty cycle accounting conduction losses:

Equation 8

RON_HS and RON_LS represent the RDSON value of the embedded power elements (see

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

where fSW is the desired switching frequency at a certain load current level.

TON

0.9 RTON CTON



VIN

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

0.9 RTON 7.5pF



VIN

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

fSW IOUT



DREAL IOUT



TON

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

DREAL IOUT



VOUT

RON_LS DCR

 I

OUT



VIN

RON_LS RON_HS

–

 I

OUT



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

RTON

0.9

--------

VIN DREAL IOUT





fSW CTON



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



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