L6986H datasheet(47/68 Pages) STMICROELECTRONICS

47 / 68 page

As anticipated above, in SLAVE mode the internal oscillator operates at 250 kHz typ. but the slope compensation

is dimensioned accordingly with FSW resistors so it is important to limit the switching operation around a working

point close to the selected oscillator frequency (FSW resistor).

As a consequence, to guarantee the full output current capability and to prevent the subharmonic oscillations, the

MASTER may limit the driving frequency range within ± 5% of the selected frequency.

A wider frequency range may generate subharmonic oscillation for duty > 50% or limit the peak current capability

(see IPK parameter in Table 1) since the internal slope compensation signal may be saturated.

The device keeps operating in slave mode as far as the master is able to drive the SYNCH pin faster than 275

kHz, otherwise the L6986H goes back into MASTER mode at the programmed RFSW oscillator frequency after

successfully driving one pulse 250 kHz typ. (see Figure 50. Slave-to-master mode transition) in the SYNCH line.

Figure 50. Slave-to-master mode transition

The external master can force a latched SLAVE mode driving the SYNCH pin low at power-up, before the soft-

start begins the switching activity. So the oscillator frequency is 250 kHz typ. fixed until a new UVLO event is

triggered regardless FSW resistor value, that otherwise counts to design the slope compensation. The same

considerations above are also valid.

The master driving capability must be able to provide the proper signal levels at the SYNCH pin (see Table 1):

•

Low level < VSYN THL = 0.7 V sinking 5 mA

•

High level > VSYN THH = 1.2 V sourcing 0.7 mA

L6986H

Synchronization (LNM)

DS12905 - Rev 2

page 47/68

L6986H датащи(PDF) 47 Page - STMicroelectronics