L6706TR datasheet(38/47 Pages) STMICROELECTRONICS

38 / 47 page

Oscillator

L6706

38/47

Doc ID 15698 Rev 2

Oscillator

The internal oscillator generates the triangular waveform for the PWM charging and

discharging with a constant current an internal capacitor. The current delivered to the

oscillator is typically 25

μA (corresponding to the free running frequency F

SW = 200 kHz)

and it may be varied using an external resistor (ROSC) connected between the OSC/FAULT

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

fixed at 1.240 V, the frequency is varied proportionally to the current sunk (forced) from (into)

the pin considering the internal gain of 10 kHz/

μA.

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

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

the pin), according the following relationships:

ROSC vs. SGND

ROSC vs. +12V

Maximum programmable switching frequency must be limited to 1 MHz to avoid minimum

Ton limitation. Anyway, device power dissipation must be checked prior to design high

switching frequency systems.

Figure 22.

ROSC vs. switching frequency

200 kHz

()

1.240V

OSC kΩ

()

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

kHz

μA

-----------

⋅

200 kHz

() 11.284 10

⋅

OSC kΩ

()

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

OSC kΩ

()

⇒

11.284 10

⋅

SW kHz

() 200 kHz

()

–

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

[]

200 kHz

() 12V 1.240V

–

OSC kΩ

()

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

kHz

μA

-----------

⋅

–

200 kHz

() 9.7916 10

⋅

OSC kΩ

()

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

–

OSC kΩ

()

⇒

9.7916 10

⋅

200 kHz

() F

SW kHz

()

–

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

[]

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