L6717ATR datasheet(47/57 Pages) STMICROELECTRONICS

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L6717A

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, F

, 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 F

=200 kHz) and it may be varied using an external resistor (R

OSC

) 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 10 kHz/

μA (See Figure 15).

Connecting R

OSC

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

according to the following relationships:

Connecting R

OSC

to a positive voltage the frequency is reduced (current is forced into the

pin), according to the following relationships:

where +V is the positive voltage which the R

OSC

resistor is connected.

Figure 15. R

OSC vs. switching frequency

200 k H z

1.240 V

OS C

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

kHz

μA

-----------

⋅

200 k H z

1.240

–

OS C

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

kHz

μA

-----------

⋅

–

100

1000

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800

900

1000

Fsw [kHz]

200

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100

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200

Fsw [kHz]

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