Circuit and description of test configurations

AN2052

6/13

DocID10909 Rev 2

By changing the resistor value between VDD and OSC pin, the frequency oscillator

(capacitor value is fixed at 220 nF) can be modified. In this way the FM percentage can

be set. The target FM percentage is +/-10%. In the original circuit a 10 kohm resistor is

used, obtaining a switching frequency of 70 kHz. This resistor value gives a charging

current capacitor of 1 mA max.

The external circuit works to increase the switching frequency. Therefore, the resistor

value should be changed between VDD and OSC pin in order to reduce t h e minimum

switching frequency to 66 kHz. As a result the new resistor value is 11 kohm, obtaining

a charging current of 0.9 mA when the external voltage is 0 V. The switching frequency

variation depends on the charging current capacitor, therefore it is proportional to the

external voltage. Maximum value of the switching frequency is 77 kHz: it is obtained

supplied from the external circuit.

Below, the design of the resistor value, between OSC pin and the oscillator, is explained:

–

in configuration “A”, maximum voltage is 374 V, so the resistance is R = 373

V/0.2 mA = 1.8 Mohm

–

in configuration “B”, the maximum voltage is 10 V, so the resistance is R = 10

V/0.2 mA = 50 kohm

It is important to verify the frequency obtained using the oscilloscope. This frequency

value has to be tuned to the desired value. In Figure 7, 8, 9 and 10 the typical clock

signal is shown.

Figure 7. Configuration “A”: AC signal (50 Hz

oscillator)

Figure 8. Configuration “A”: PWM ramp signal