SNVS726F July 2011 – March 2018 LM25118
PRODUCTION DATA.
Each conduction cycle of the buck switch is followed by a forced minimum off-time of 400 ns to allow sufficient time for the recirculating diode current to be sampled. This forced off-time limits the maximum duty cycle of the controller. The actual maximum duty cycle will vary with the operating frequency of Equation 7.
where
Limiting the maximum duty cycle will limit the maximum boost ratio (VOUT/VIN) while operating in buck-boost mode. For example, from Figure 15, at an operating frequency of 500 kHz, DMAX is 80%. Using the buck-boost transfer function.
with
With a minimum input voltage of 5 V, the maximum possible output voltage is 20 V at f = 500 kHz. The buck-boost step-up ratio can be increased by reducing the operating frequency which increases the maximum duty cycle.