15/35

AN628 APPLICATION NOTE

Figure 17.

Pin 16. SYNC (In/Out synchronization). Only for L4981A, this function allows the device to be synchronized with

other circuits of a system (see fig.18a). When the device is externally synchronized, the external clock has to

satisfy these conditions: the signal amplitude must cross the threshold value (3.5V), the frequency has to be

slightly higher than that programmed by the R-C constant (see pin 18) and the pulse width has to be at least

800 nsec.

If the device has to synchronize other circuits, the signal delivered by this pin is a positive pulse of 4.6V (0.5mA)

and the pulse duration is equal to the sawtooth falltime.

The L4981B uses this pin to perform another function. If the application does not use the SYNC function, it is

preferrable to focus the EMI filtering problem using the B version. Pin 16, named FREQ-MOD in B version, al-

lows to change the switching frequency in order to spread the energy content over a wider spectrum range.

currents that define the ramp slopes of the oscillator sawtooth. The effect of the resistor produces the frequency

the formula:

+

-

UVLO

1.28V

19

P-UVLO

15

10

D94IN066A

RH

RL

11V

10V

82k

Ω

12k

Ω

12V

10.1V

220k

Ω

33k

Ω

13V

10.5V

430k

Ω

62k

Ω

14V

10.8V

909k

Ω

133k

Ω

14.5V

10.9V

1.36M

Ω

200k

Ω

15V

11V

2.7M

Ω

390k

Ω

a)

b)

Programmable Under Voltage

f

sw

∆

f

sw

-----------

K

V

IPK

R

osc

⋅

V

RMS

R

fm

⋅

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

⋅

=