L6714 datasheet(54/70 Pages) STMICROELECTRONICS

54 / 70 page

Oscillator

L6714

54/70

Oscillator

L6714 embeds four phase oscillator with optimized phase-shift (90º phase-shift) in order to

reduce the input rms current and optimize the output filter definition.

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 so that the resulting switching frequency at the

load side results in being multiplied by N (number of phases).

The current delivered to the oscillator is typically 25

µA (corresponding to the free running

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

between the OSC pin and SGND or VCC (or a fixed voltage greater than 1.24V). Since the

OSC pin is fixed at 1.24V, the frequency is varied proportionally to the current sunk (forced)

from (into) the pin considering the internal gain of 6KHz/

µA.

In particular connecting ROSC to SGND the frequency is increased (current is sunk from the

pin), while connecting ROSC to VCC = 12V the frequency is reduced (current is forced into

the pin), according the following relationships:

ROSC vs. SGND

ROSC vs. +12V

When using the Low-Side MOSFETs current sense, the maximum programmable switching

frequency per phase must be limited to 500kHz to avoid current reading errors causing, as a

consequence, current sharing errors.

Anyway, device power dissipation must be checked prior to design high switching frequency

systems.

150 kHz

()

1.240V

OSC kΩ

()

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

kHz

µA

-----------

⋅

150 kHz

()

7.422

⋅

OSC kΩ

()

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

OSC k

Ω

()

⇒

-----------

µA

---------

150 kHz

()

7.422

⋅

OSC kΩ

()

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

OSC k

Ω

()

⇒

7.422

⋅

SW kHz

() 150 kHz

()

–

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

Ω

[]

150 kHz

()

12V

1.240V

–

OSC kΩ

()

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

kHz

µA

-----------

⋅

–

150 kHz

() 6.456 10

⋅

OSC kΩ

()

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

–

OSC kΩ

()

⇒

-----

kHz

µA

-----------

150 kHz

() 6.456 10

⋅

OSC kΩ

()

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

–

OSC kΩ

()

⇒

6.456

⋅

150 kHz

() F

SW kHz

()

–

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

Ω

[]

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