L6717TR datasheet(46/56 Pages) STMICROELECTRONICS

46 / 56 page

Main oscillator

L6717

46/56

Doc ID 17326 Rev 1

Main oscillator

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

phase 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 200 kHz: 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 20

μA (corresponding to the free running

frequency FSW=200 kHz) 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 1.240 V, the

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

gain of 9.1 kHz/

μA (See Figure 15).

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

according to the following relationships:

Connecting ROSC to a positive voltage (recommended 3.3 V rail) the frequency is reduced

(current is injected into the pin), according to the following relationships:

where +V is the positive voltage which the ROSC resistor is connected.

Figure 15.

ROSC vs. switching frequency

200kHz

1.240V

OSC

------------------- 9.1

kHz

μA

-----------

⋅

200kHz

1.240

–

OSC

---------------------------- 9.1

kHz

μA

-----------

⋅

–

FSW vs. Rosc

125

175

225

275

325

375

425

100

150

200

250

300

350

400

450

Rosc [kOhm]

Rosc+ (to SGND)

Rosc- (to 3V3)

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