L6739 datasheet(16/30 Pages) STMICROELECTRONICS

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Main oscillator

L6739

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DocID026384 Rev 1

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

A (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/

A (see Figure 7).

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

according to the following relationships:

Equation 6

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

pin), according to the following relationships:

Equation 7

where Vj is the positive voltage to which the ROSC resistor is connected.

Figure 7. ROSC vs. switching frequency

FSW

200kHz

1.240V

ROSC

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

kHz

A

-----------



FSW

200kHz

Vj 1.240

–

ROSC

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

kHz

A

-----------







–

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