L6722 datasheet(26/34 Pages) STMICROELECTRONICS

26 / 34 page

10 Oscillator

L6722

26/34

Oscillator

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 100kHz so that the resulting switching frequency at the load

side results in being tripled (300kHz).

The current delivered to the oscillator is typically 25

µA (corresponding to the free running

frequency FSW=100kHz) and it may be varied using an external resistor (ROSC) connected

between the OSC pin and SGND. Since the OSC pin is fixed at 1.24V, the frequency is varied

proportionally to the current sunk from the pin considering the internal gain of 4KHz/

µA.

In particular connecting ROSC to SGND the frequency is increased according to the following

relationship:

ROSC vs. SGND

Caution:

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.

Figure 15. ROSC vs. Switching frequency

100kHz

1.240V

OSC kΩ

()

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

kHz

µA

-----------

⋅

100kHz

4.96

⋅

OSC kΩ

()

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

100

150

200

250

100

150

200

250

300

350

400

450

500

550

Fsw [kHz] Programmed

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