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AN2052 Datasheet(PDF) 6 Page - STMicroelectronics |
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AN2052 Datasheet(HTML) 6 Page - STMicroelectronics |
6 / 13 page 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 when the charging current is 1.1 mA where 0.9 mA is given by VDD and 0.2 mA is 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 |
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