8.6 Bootstrap Charging and Maximum Duty Cycle Limitations

The bootstrap capacitor supplies current to the internal high-side drive circuitry that is above the input voltage of the converter. It must store enough energy to completely drive the high-side switch on and off. A 0.1 µF X5R or X7R capacitor is recommended for all applications. The bootstrap capacitor maximum voltage is 5.5V; therefore, a capacitor with a voltage rating of 6.3V or 10V is recommended.

The bootstrap capacitor is charged during the off-time of the switching cycle, when the SW node is pulled to GND through the BD pin. When operating at a low voltage difference between input and output, the duty cycle can reach its limit of approximately 87%. Combined with the high switching frequency of 2.2 MHz, this results in a charging window of only 50 ns for the bootstrap capacitor.

In most cases, this 50 ns interval is sufficient to replenish the energy lost during each switching cycle. However, when the voltage applied on the BD pin is below 3V and the maximum duty cycle is reached, the voltage on the bootstrap capacitor can decrease, reaching 2V and forcing an internal charge of the bootstrap capacitor and, therefore, a stop in switching activity.

To enhance performance at low input voltages, where the boost charge time reaches 50 ns, the MCP16367/8/9 further limits the maximum duty cycle to 75% typical when the input voltage is below 5.1V. This adjustment allows the device to continue switching and operating correctly, albeit with a greater margin between V_IN and V_OUT. When V_IN exceeds 6V, the maximum duty cycle returns to 87% for normal operation.

When the maximum duty cycle operation is expected to improve the operation at low V_IN, an ultra-fast external bootstrap diode can be connected to the boost pin to improve the charging of the bootstrap capacitor.