Datasheet

L7985 Functional description
Doc ID 022446 Rev 3 9/43
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 connect 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 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 pin together. When the SYNCH pins are connected, the device with
a higher oscillator frequency works as 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).
Figure 4. Oscillator circuit block diagram
The device can be synchronized to work at higher frequency feeding an external clock
signal. The synchronization changes the sawtooth amplitude, changing the PWM gain
(Figure 5.c). This change has to 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