L6751C datasheet(44/57 Pages) STMICROELECTRONICS

44 / 57 page

Main oscillator

L6751C

44/57

DocID025037 Rev 1

Main oscillator

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

discharging, with a constant current, on the internal capacitor. 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

A 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/

A for the multi-phase section and

of 11.5 kHz/

A for the single-phase section, see Figure 14.

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

Figure 14. ROSC vs. FSW per phase (ROSC to GND - left; ROSC to 3.3 V - right)

FSW

200kHz

1.02V

ROSC k



---------------------------- 10

kHz

A

-----------



FSSW

250kHz

1.02V

RSOSC k



-------------------------------- 11.5

kHz

A

-----------



FSW

200kHz

Vbias 1.02V

–

ROSC k



-------------------------------------- 10

kHz

A

-----------



–

FSSW

250kHz

Vbias 1.02V

–

RSOSC k



-------------------------------------- 11.5

kHz

A

-----------



–

100

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Multi Phase section

SinglePhase section

100

1000

100

125

150

175

200

225

Multi Phase section

SinglePhase section

AM14820v1

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