L6713ATR datasheet(50/64 Pages) STMICROELECTRONICS

50 / 64 page

Oscillator

L6713A

50/64

Oscillator

L6713A embeds two/three phase oscillator with optimized phase-shift (180º/120º phase-

shift) in order to reduce the input rms current and optimize the output filter definition.

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

discharging with a constant current an internal capacitor. The switching frequency for each

channel, FSW, is internally fixed at 200 kHz so that the resulting switching frequency at the

load side results in being multiplied by N (number of phases).

The current delivered to the oscillator is typically 25

μA (corresponding to the free running

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

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

OSC pin is fixed at 1.24 V, the frequency is varied proportionally to the current sunk (forced)

from (into) the pin considering the internal gain of 6 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 per phase 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 24.

ROSC vs. switching frequency

200 kHz

()

1.240V

OSC kΩ

()

---------------------------- 6

kHz

μA

-----------

⋅

200 kHz

() 7.422 10

⋅

OSC kΩ

()

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

OSC kΩ

()

⇒

kHz

μA

-----------

200 kHz

() 7.422 10

⋅

OSC kΩ

()

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

OSC kΩ

()

⇒

7.422 10

⋅

SW kHz

() 200 kHz

()

–

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

[]

200 kHz

() 12V 1.240V

–

OSC kΩ

()

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

8kHz

μA

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

⋅

–

200 kHz

() 8.608 10

⋅

OSC kΩ

()

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

–

OSC kΩ

()

⇒

kHz

μA

-----------

200 kHz

() 6.456 10

⋅

OSC kΩ

()

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

–

OSC kΩ

()

⇒

8.608 10

⋅

200 kHz

() F

SW kHz

()

–

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

[]

1000

2000

3000

4000

5000

6000

7000

100

125

150

175

200

Fsw [kHz] Programmed

100

150

200

250

300

350

400

150

250

350

450

550

650

750

850

950

1050

Fsw [kHz] Programmed

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