L6730C datasheet(13/50 Pages) STMICROELECTRONICS

13 / 50 page

L6730C - L6730D

Device description

13/50

Device description

5.1

Oscillator

The switching frequency is internally fixed to 400KHz. The internal oscillator generates the

triangular waveform for the PWM charging and discharging an internal capacitor (FSW =

400KHz). This current can be varied using an external resistor (RT) connected between OSC

pin and GND or VCCDR in order to change the switching frequency. Since the OSC pin is

maintained at fixed voltage (typ. 1.2V), the frequency is increased (decreased) proportionally to

the current sunk (sourced) from (into) the pin. In particular, connecting RT versus GND the

frequency is increased (current is sunk from the pin), according to the following relationship:

Connecting RT to VCCDR the frequency is reduced (current is sourced into the pin), according to

the following relationship:

Switching frequency variation vs. RT is shown in Figure 3..

Figure 3.

Switching frequency variation versus RT.

)

(

400

Ω

⋅

KHz

Fsw

OSC

(1)

)

(

400

Ω

⋅

−

KHz

Fsw

OSC

(2)

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

100

200 300

400 500 600

700 800

900 1000

Rosc (KOHM)

Rosc connected to GND

Rosc connected to Vccdr

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