L6759D datasheet(37/51 Pages) STMICROELECTRONICS

37 / 51 page

L6759D

Main oscillator

Doc ID 023240 Rev 1

37/51

Main oscillator

The internal oscillator generates the triangular waveform for the PWM charging and

discharging an internal capacitor with a constant current. The switching frequency for each

channel is internally fixed at 200 kHz (FSW) and at 230 kHz (FSSW): the resulting switching

frequency at the load side for the multi-phase section results in being multiplied by N

(number of configured phases).

The current delivered to the oscillator is typically 20

µA and may be varied using an external

resistor (ROSC, RSOSC) typically connected between the OSC, SOSC pins and GND. Since

the OSC/SOSC pins are fixed at 1.02 V, the frequency is varied proportionally to the current

sunk from the pin considering the internal gain of 10 kHz/

µA for the multi-phase section and

of 11.5 kHz/

µA for the single-phase section, see Figure 14.

By connecting ROSC/RSOSC to SGND the frequency is increased (current is sunk from the

pin), according to the following relationships:

Equation 17

Equation 18

Connecting ROSC/RSOSC to a positive voltage Vbias, the frequency is reduced (current is

injected into the pin), according to the following relationships:

Equation 19

Equation 20

200kHz

1.02V

OSC kΩ

()

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

kHz

µA

-----------

⋅

SSW

230kHz

1.02V

SOSC kΩ

()

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

11.5

kHz

µA

-----------

⋅

200kHz

Vbias

1.02V

–

OSC kΩ

()

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

kHz

µA

-----------

⋅

–

SSW

230kHz

Vbias

1.02V

–

SOSC kΩ

()

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

11.5

kHz

µA

-----------

⋅

–

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