8.6 Bootstrap Charging and Maximum Duty Cycle Limitations

The bootstrap capacitor is used to supply current for the internal high-side drive circuitry that is above the input voltage of the converter. The bootstrap capacitor 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, so a 6.3V- or 10V-rated capacitor is recommended.

The charging of the bootstrap capacitor is done 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 will reach the maximum limitation at around 87%. This, coupled with the high switching frequency of 2.2 MHz, allows for only 50 ns of charging time for the bootstrap capacitor.

In most cases, this 50 ns time is sufficient to replenish the energy lost for the switching activity during each 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 improve the behavior 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 for input voltages below 5.1V. Due to this, the parts can continue switching and operating correctly, although with a larger headroom between V_IN and V_OUT. When the V_IN recovers and is above 6V, the maximum duty cycle is again set 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.