L6711 datasheet(27/38 Pages) STMICROELECTRONICS

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L6711

mosfet of that phase until IINFOx re-cross the threshold or until the next clock cycle. This implies that the

device limits the peak of the inductor current.

In any case, the inductor current won't overcome the IOCPx value and this will represent the maximum peak

value to consider in the OC design.

The device works in Constant-Current, and the output voltage decreases as the load increase, until the

output voltage reaches the UVP threshold. When this threshold is crossed, all mosfets are turned off, the

FAULT pin is driven high and the device stops working. Cycle the power supply or the OUTEN pin to re-

start operation.

The transconductance resistor Rg can be designed considering that the device limits the inductor current

ripple peak. Moreover, when designing D-VID systems, the additional current due to the output filter

charge during dynamic VID transitions must be considered.

where

14 OSCILLATOR

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 150kHz so that the resulting switching frequency at the load side results in being tripled.

The current delivered to the oscillator is typically 25

µA (corresponding to the free running frequency

Fsw=150kHz) 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 1.24V). Since the OSC pin is fixed at 1.24V, the frequency is

varied proportionally to the current sunk (forced) from (into) the pin considering the internal gain of 6KHz/

µA.

In particular connecting ROSC to SGND the frequency is increased (current is sunk from the pin), while

connecting ROSC to VCC=12V the frequency is reduced (current is forced into the pin), according to the

following relationships:

Maximum programmable switching frequency depends on the Current Reading Method selected. When

reading across LS mosfet, the maximum switching frequency per phase must be limited to 500kHz to

avoid current reading errors causing, as a consequence, current sharing errors. When reading across the

inductor, higher switching frequency can be approached (device power dissipation must be checked prior

to design high switching frequency systems).

Figure 20. ROSC vs. Switching Frequency

OCPx max

() RSENSE max

()

⋅

OCTH min

()

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

OCPx

OUT OCP

()

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

∆

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

D-VID

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

OSC vs. GND: FSW

150kHz

1.237

OSC kΩ

()

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

kHz

µA

-----------

⋅

150kHz

7.422 10

⋅

OSC k Ω

()

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

OSC vs. 12V: FSW

150 kHz

12-1.237

OSC kΩ

()

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

kH z

µA

-----------

⋅

150kHz

6.457 10

⋅

OSC k Ω

()

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

2000

4000

6000

8000

10000

12000

14000

100

125

150

200

400

600

800

1000

150

250

350

450

550

650

750

850

ROSC [k

Ω] to 12V vs. Selected FSW [kHz]

ROSC [k

Ω] to SGND vs. Selected FSW [kHz]

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