AN469 APPLICATION NOTE

4/6

SYNCHRONIZING MULTIPLE DEVICES

Ground noise problems in multiple configurations can be avoided by synchronizing the oscillators. This may be

done by connecting the sync pins of each of the devices with the oscillator output of the master device and con-

necting the R/C pin of the unused oscillators to ground as shown in figure 5.

The devices may be synchronized to external circuits by applying synchronizing pulses to the sync pins. It

should be noted, however, that the input pulse sets the minimum on time of the outputs and will therefore set a

minimum output average current.

SELECTING THE OSCILLATOR COMPONENTS

When selecting the values for the external components for the oscillator one of the primary considerations is the

operating frequency. In addition there is another important consideration for these components.

In many applications the reverse recovery current of the free wheeling diodes and of parasitic elements in the

power stage will flow through the sensing resistor in addition to the load current. Also there is sometimes noise

generated in the system when the power stage is swiched on. These two sources of error can fool the current

limiting stage and make it appear to operate at a subharmonic of the desired frequency. With the proper selec-

tion of the oscillator components this behavior can be avoided.

The design of the L6506 is such that the flip-flops used in the device are set dominant so that whenever the sync

input is low the Q output of the flip-flop will be high even if the reset is applied by the comparator at the same

time. This characteristic of the flip-flops can be used to make the current sensing immune to the recovery cur-

rents and noise spikes that occur when the power devices switch. If the sync pulse is longer than the turn on

delay time of the power stage, as shown in figure 6, these two sources of errors will be ignored.

To select the proper values for the oscillator components a more detailed equation for the operating frequency

and duty cycle of the oscillator is required.

The required equations can be derived from the equivalent circuit for the oscillator section shown in figure 7.

As can be seen from figure 7, the full equation for the operating frequency includes not only the external resis-

tance and capacitance but the internal discharge resistor as well.

The full equation for the operating frequency is:

:

(3)

The equations for the active time of the sync pulse (T2), the inactive time of the sync signal (T1) and the duty

cycle can also be found by looking at the figure 7 and are :

(4)

T1 = 0.69 R1 C1

(5)

(6)

By substituting equations 4 and 5 into equation 6 and solving for the value of R1 the following equations for the

external components can be derived:

(7)

f

1

0.69C1 R1

R1 Ri

⋅

R1

Ri

+

--------------------





+

--------------------------------------------------------------------

=

T2

0.69C1

R1Ri

R1

Ri

+

--------------------

=

DC

T2

T1

T2

+

---------------------

=

R1

1

DC

---------

2

–



 Ri

=