L6738ATR datasheet(19/32 Pages) STMICROELECTRONICS

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L6738A

Main oscillator

Doc ID 18134 Rev 2

19/32

Main oscillator

The controller embeds a programmable oscillator. The internal oscillator generates the

sawtooth waveform for the PWM charging with a constant current and resetting an internal

capacitor. The switching frequency, FSW, is internally fixed at 200 kHz.

The current delivered to the oscillator is typically 20

µA (corresponding to the free running

frequency FSW=200 kHz) and it may be varied using an external resistor (ROSC) typically

connected between the OSC pin and GND. As the OSC pin is fixed at 1.240 V, the

frequency is varied proportionally to the current sunk from the pin considering the internal

gain of 10 KHz/

µA (see Figure 7).

Connecting ROSC to GND, the frequency is increased (current is sunk from the pin),

according to the following relationships:

Connecting ROSC to a positive voltage, the frequency is reduced (current is forced into the

pin), according to the following relationships:

where +V is the positive voltage to which the ROSC resistor is connected.

Figure 7.

ROSC vs. switching frequency

200kHz

1.240V

OSC

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

kHz

µA

-----------

⋅

200kHz

1.240

–

OSC

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

kHz

µA

-----------

⋅

–

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