7.3.4 Oscillator and PLL

The switching frequency is adjustable over a range from 100 kHz to 600 kHz by placing a resistor on the RT pin. The RT pin voltage is typically 0.5 V and must have a resistor to ground to set the switching frequency. To determine the timing resistance for a given switching frequency, use Equation 2 or the curve in Figure 3. To reduce the solution size one would typically set the switching frequency as high as possible, but give consideration to tradeoffs of the supply efficiency, maximum input voltage, and minimum controllable on-time.

Equation 2.

One can also use the RT pin to synchronize the controllers to an external system clock, over a range from 100 kHz to 600 kHz. Apply a square wave to the RT pin to use this synchronization feature. The square wave must transition lower than 0.8 V and higher than 2 V on the RT pin and have an on-time greater than 70 ns and an off time greater than 70 ns. The synchronization frequency range is 100 kHz to 600 kHz. The rising edge of GDRV1 is synchronized to the falling edge of the RT pin signal.

Leaving the RT pin open or shorted to ground with no external system clock signal is present disables both boost controllers, and both PWM dimming FETs switch off. In order to recover from this global failure state, (for example, after the failure condition on the RT pin has been removed) there must be one global disable-and-enable cycle (active shutdown by pulling both PWMINx pins low for t > t_{(CH_OFF)}, and setting one or both PWMINx pins high for t > t_{(CH_ON)}).