UC3525A: Regarding RT, CT and RD

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 Michiaki,

The device uses an external resistor to set the charging current for the external capacitor, which determines the oscillator frequency. TI 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