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L6918

5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER

STMicroelectronics 

L6918 L6918A

– Clock Signal (SYNC_IN / SYNC_OUT pins): Unidirectional line.

A synchronization signal exits from the Master device through the SYNC_OUT pin with 90 deg phase-

shift and enters the Slave device through the SYNC_IN pin. The Slave device locks that signal

through an internal PLL for its regulation. An auxiliary synchronization signal exits from the Slave

through the SYNC_OUT.

– SLAVE_OK Bus (SLAVE_OK pins): Bi-directional line.

While the supply voltages are increasing, this line is hold to GND by all the devices. The Slave device

sets this line free (internally 5V pulled-up) when it is ready for the Soft-Start. After that this line is

freed, the Master device starts the Soft Start (for further details about Soft-Start, see the relevant sec-

tion).

During normal operation, the line is pulled low by the Slave device if an Over / Under voltage is de-

tected (See relevant section).

– PGOOD pins:

PGOOD pins are connected together and pulled-up. During Soft-Start, the master device hold down

this line while during normal regulation the slave device de-assert the line if PGOOD has been lost.

Connections between the devices are shown in figure 1.

OSCILLATOR

The devices have been designed in order to operate on each phase at the same switching frequency of the in-

ternal oscillator. So, input and output resulting frequencies are four times bigger.

The oscillator is present in all the devices. Since the Master oscillator sets the main frequency for the regulation,

the Slave oscillator gives an offset to the Slave's PLL. In this way the PLL is able to lock the synchronization

signal that enters from its SYNC_IN pin; it is able to recover up to ±15% offset in the synchronization signal fre-

quency. It is then necessary to program the switching frequency for all the devices involved in the multi-phase

conversion as follow.

The switching frequency is internally fixed to 300kHz. The internal oscillator generates the triangular waveform

for the PWM charging and discharging with a constant current an internal capacitor. The current delivered to the

oscillator is typically 25µA (Fsw = 300KHz) and may be varied using an external resistor (ROSC) connected be-

tween OSC pin and GND or Vcc. Since the OSC pin is maintained at fixed voltage (typ. 1.235V), the frequency

is varied proportionally to the current sunk (forced) from (into) the pin considering the internal gain of 12KHz/µA.

In particular connecting it to GND 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 relation-

ships:

ROSC vs. GND: fS = 300kHz + R-----O---1-S--.--C2---3(---K-7---Ω-----)- ⋅ 12 K---µ--H--A---z- = 300KHz + -R1---4-O--.--S8---2C----(-⋅--K-1---Ω-0---6-)-

ROSC vs. 12V: fS = 300kHz + R--1---2O----S-–---C-1--(--.-K2----3Ω---7---) ⋅ 12 K---µ--H--A---z- = 300KHz – -1-R--2--O-.--9-S--1--C--8--(--K⋅---1--Ω--0---)7-

Note that forcing a 25µA current into this pin, the device stops switching because no current is delivered to the

oscillator.

Figure 2 shows the frequency variation vs. the oscillator resistor ROSC considering the above reported relation-

ships.

11/35

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