Datasheet

DocID022450 Rev 7 9/45
L7986 Functional description
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5.1 Oscillator and synchronization
Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides
a constant frequency clock. Its frequency depends on the resistor externally connected to
the FSW pin. If the FSW pin is left floating, the frequency is 250 kHz; it can be increased as
shown in Figure 6 by an external resistor connected to ground.
To improve the line transient performance, keeping the PWM gain constant versus the input
voltage, the voltage feed-forward is implemented by changing the slope of the sawtooth
according to the input voltage change (see Figure 5.a).
The slope of the sawtooth also changes if the oscillator frequency is increased by the
external resistor. In this way a frequency feed-forward is implemented (Figure 5.b) in order
to keep the PWM gain constant versus the switching frequency (see Section 6.4 on page 18
for PWM gain expression).
On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of
180° with respect to the clock. This delay is useful when two devices are synchronized
connecting the SYNCH pins together. When SYNCH pins are connected, the device with
a higher oscillator frequency typically works as a master, so the slave device switches at the
frequency of the master but with a delay of half a period. This minimizes the RMS current
flowing through the input capacitor (see the L5988D datasheet).
The SYNCH circuitry is also able to synchronize with a slightly lower external frequency, so
the frequency pre-adjustment with the same resistor on the FSW pin, as described below, is
suggested for a proper operation.
Figure 4. Oscillator circuit block diagram
The device can be synchronized to work at a higher frequency feeding an external clock
signal. The synchronization changes the sawtooth amplitude, changing the PWM gain
(Figure 5.c). This change must be taken into account when the loop stability is studied. To
minimize the change of PWM gain, the free-running frequency should be set (with a resistor
on the FSW pin) only slightly lower than the external clock frequency. This pre-adjusting of
the frequency changes the sawtooth slope in order to render the truncation of sawtooth
negligible, due to the external synchronization.
Clock
Generator
Ramp
Generator
FSW
Sawtooth
Clock
Synchronization
SYNCH
Clock
Generator
Ramp
Generator
FSW
Sawtooth
ClockClock
Synchronization
SYNCH