Semiconductor Group

9

PSB 2121

At the beginning of the discharge period a positive synchronization pulse is generated at pin SYNC.

Otherwise the PSB 2121 can be synchronized via pin SYNC to an external logic clock by

programming the oscillator to free run at a frequency 10 % lower than the synchronization

frequency. The PSB 2121 is synchronized by the rising edge of the sync. signal. So multiple devices

can be synchronized together by programming one master unit for the desired frequency.

Notice that the frequency of the output driver is half the oscillator frequency. The switching

frequency as a function of

R

Figure 3

Switching Frequency

Soft Start Circuit

The soft start circuit protects the power transistors and rectifier diodes from high current surges

during power supply turn-on. When the supply voltage is connected to the PSB 2121 the

undervoltage lockout circuit holds the soft start capacitor voltage at zero. When the supply voltage

reaches normal operating range an internal 4

µA current source will charge the external soft start

capacitor. As the soft start voltage ramps up to + 5 volts, the duty cycle of the PWM linearly

increases to whatever value the regulation loop requires.