L6758ATR datasheet(34/42 Pages) STMICROELECTRONICS

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Main oscillator

L6758A

34/42

DocID023298 Rev 2

Main 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, F

, F

SSW

, is internally fixed at 200 kHz: the resulting switching frequency at the

load side for the multi-phase section results in being multiplied by N (number of configured

phases).

The current delivered to the oscillator is typically 20

μA (corresponding to the freerunning

frequency F

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

OSC

, R

SOSC

)

typically connected between the OSC, SOSC pins and GND. Since the OSC/SOSC pins are

fixed at 1 V, the frequency is varied proportionally to the current sunk from the pin

considering the internal gain of 10 KHz/

μA for F

or 11 KHz/

μA for F

SSW

(see

Figure 10).

Connecting R

OSC

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

according to the following relationships:

Equation 12

Figure 10. ROSC [kΩ] vs. switching frequency [kHz] per phase.

200 kH z

1.000 V

OS C

()

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

kH z

μA

-----------

⋅

AM11143v1

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