6

UC1826

UC2826

UC3826

Figure 1. Oscillator Block with External Connections

PWM Oscillator: The chip has two pins that set RC time

constants. The resistor and capacitor tied to RAMP cre-

ate the ramp used as the input to the PWM comparator.

When the output pin OUT is high, RAMP charges until it

passes the PWM comparator threshold. The output is

then driven low and RAMP is discharged. The resistors

and capacitor on the OSC pin are used to set the PWM

operating frequency and its maximum duty cycle.

The oscillator block diagram with external wiring is shown

the exponential charge current. The oscillator charges

from 1.2V to a 3.4V threshold with an RC time delay of 2

RS flip-flop is set driving CLKSYN high and RDEAD low

RDEAD with a RC time delay of 2

cillator and ramp waveforms are shown in Figure 2.

Equations to attain frequency and maximum duty cycle

are listed under the OSC pin description.

As shown in Figure 3, several oscillators are synchro-

nized to the highest free running frequency by connect-

ing 100pF capacitors in series with each CLKSYN pin

and connecting the other side of the capacitors together

forming the CLKSYN bus. The CLKSYN bus is then

pulled down to ground with a resistance of approximately

10k. Referring to Figure 1, the synchronization threshold

is 1.4V. The oscillator blanks any synchronization pulse

that occurs when OSC is below 2.5V. This allows units,

once they discharge below 2.5V, to continue through the

current

discharge

and

subsequent

charge

cycles

whether or not other units on the CLKSYN bus are still

synchronizing. This requires the frequency of all free run-

ning oscillators to be within 40% of each other to guaran-

tee synchronization.

Grounds, Voltage Sensing and Current Sensing: The

voltage is sensed directly at the load. Proper load sharing

requires the same sensed voltage for each power supply

connected

in

parallel. Referring

to

Figure

4,

the

CIRCUIT DESCRIPTION:

OSC

OUT

CLKSYN

RAMP

CAO

3.0-

1.0

Figure 2. Oscillator and PWM Output Waveform

UDG-95014-1