data/text/DLP3/898303

UC3525A: Regarding RT, CT and RD

Part Number: UC3525A Hi team, Could you kindly advise the below inquires form customer ? 1. Why the charge time at CT 0.1uF in Fig 1 is longer than at CT 0.02uF ? It seems the number of CT get be smaller and smaller and charge time get be longer as well. 2. What is charge time ? - What should customer consider from Fig 1 ? - It looks different that charge time is switching frequency since switching frequency is provided by equation f = 1/CT(0.7RT + 3RD) Is the time until full charge of CT ? 3. Regarding fig 2 What can customer find from Fig 2 ? When customer configured RD = 100 ohm, how long will dead time take ? 4. Could you kindly provide the behavior if sync is used ? Customer would like to know the relation between internal clock and external clock. Thank you and best regards, Michiaki

Hi Machiaki, 1./ It should be a writing mistake in figure 1, it is 0.1uf rather than 0.01uf. With the same RT, the larger CT, the longer charger time. 2./ Figure 1 is the charge time, not switching period, the switching period should be the sum of charging time and discharging time. The equation f = 1/CT(0.7RT + 3RD) can be used to approximately calculated the frequency. 3./ According figure 2, if CT is defined, then we can configure RD to get the desired dead time (discharging time). 4./ If synchronization is used, the sync signal will force to arrive the internal discharging threshold earlier. The PWM is allowed to run at the frequency set by RT, CT and RD until the sync pulse appears. Generally, the free frequency must be set to a lower frequency than the sync pulse, typically 15%. Regards, Teng

Hi Teng, Could you kindly advise how we can see the figure 1? Thank you and best regards, Michiaki

Hi Michiaki, The device uses an external resistor to set the charging current for the external capacitor, which determines the oscillator frequency.company recommends timing resistor values from 2 kΩ to 150 kΩ and timing capacitor values from 0.001 uF to 0.01 uF. See Figure 1 for component values for setting the charging time (oscillator frequency). And see Figure 2 for dead time resister value for setting the discharging time (dead time), and dead time resister value from 0 to 500 Ω. Because resistors are available in more precise increments, typically 1%, and capacitors are only available in 5% accuracy, it might be more practical to select the closest capacitor value first and then select the timing resistor value next according the figure 1 and figure 2. Regards, Teng

Hi Teng, Could you kindly advise why the charge time is longer as CT is smaller ? Thank you and best regards, Michiaki

Hi Michiaki, The charging time should be the larger, as the CT increases. There is a mistake in figure 1, please exchange the position of 0.01 uF and 0.1 uF. Regards, Teng