L6730B datasheet(14/52 Pages) STMICROELECTRONICS

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Device description

L6730 - L6730B

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Doc ID 11938 Rev 3

Device description

5.1

Oscillator

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

triangular waveform for the PWM charging and discharging an internal capacitor (FSW = 400

kHz). 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. 1.2 V), the frequency is increased (or decreased)

proportionally to the current sunk (sourced) from (into) the pin. In particular by connecting

RT versus GND the frequency is increased (current is sunk from the pin), according to the

following relationship:

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

to the following relationship:

Switching frequency variation vs. RT is shown in Figure 4.

Figure 4.

Switching frequency variation versus RT

)

(

400

⋅

KHz

Fsw

OSC

(1)

)

(

400

⋅

−

KHz

Fsw

OSC

(2)

100

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1400

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Rosc (KOHM)

Rosc connected to GND

Rosc connected to Vccdr

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