Detailed Pin Functions Description
Pin 1. SYNC (In/Out Synchronization). This func-
tion allows the IC’s oscillator either to synchronize
other controllers (master) or to be synchronized to
an external frequency (slave).
As a master, the pin delivers positive pulses dur-
ing the ramp-down of the oscillator (see pin 2). In
slave operation the circuit is edge triggered. Refer
to fig. 21 to see how it works. When several IC
Figure 20. Synchronizing the L4990.
L4990 - L4990A
work in parallel no master-slave designation is
needed because the fastest one becomes auto-
matically the master.
During the ramp-up of the oscillator the pin is
pulled low by a 600µA generator. During the
ramp-down, that is when the pulse is released,
the 600µA pull-down is disconnected. The pin be-
comes a generator whose source capability is
typically 7mA (with a voltage still higher than
In fig. 20, some practical examples of synchroniz-
ing the L4990 are given.
2 L4990 1
16 17 18
Pin 2. RCT (Oscillator). A resistor (RT) and a ca-
pacitor (CT), connected as shown in fig. 21 set the
operating frequency fosc of the oscillator.
CT is charged through RT until its voltage reaches
3V, then is quickly internally discharged. As the
voltage has dropped to 1V it starts being charged
Figure 21. Oscillator and synchronization internal schematic.