L6740L datasheet(36/44 Pages) STMICROELECTRONICS

36 / 44 page

Main oscillator

L6740L

36/44

Main oscillator

The controller embeds a dual-Oscillator: one section is used for the CORE and it is a

multiphase programmable oscillator managing equal phase-shift among all phases and the

other section is used for the NB section. Phase-Shift between the CORE and NB ramps is

automatically adjusted according to the CORE phase # programmed.

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

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

channel, FSW, is internally fixed at 150kHz: the resulting switching frequency for the CORE

section at the load side results in being multiplied by N (number of configured phases).

The current delivered to the oscillator is typically 22

µA (corresponding to the free running

frequency FSW=150kHz) and it may be varied using an external resistor (ROSC) typically

connected between the OSC pin and SGND. Since the OSC pin is fixed at 1.240V, the

frequency is varied proportionally to the current sunk from the pin considering the internal

gain of 6.8KHz/

µA (See Figure 15).

Connecting ROSC to SGND the frequency is increased (current is sunk from the pin),

according to the following relationships:

Figure 15.

ROSC vs. switching frequency

150kHz

1.240V

OSC kΩ

()

---------------------------- 6.8

kHz

µA

-----------

⋅

150kHz

8.432 10

⋅

OSC kΩ

()

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

100

200

300

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500

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800

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1000

100

1000

Rosc Resistor [kOhms]

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