7.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)

The TPS54560B provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and SW pins provides the gate-drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the high-side MOSFET is off and the external low-side diode conducts. The recommended value of the BOOT capacitor is 0.1 μF. TI recommends a ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher for stable performance over temperature and voltage.

When operating with a low voltage difference from input to output, the high-side MOSFET of the TPS54560B operates at 100% duty cycle as long as the BOOT to SW pin voltage is greater than 2.1 V. When the voltage from BOOT to SW drops below 2.1 V, the high-side MOSFET is turned off, and an integrated low side MOSFET pulls SW low to recharge the BOOT capacitor. To reduce the losses of the small low-side MOSFET at high output voltages, it is disabled at 24-V output and re-enabled when the output reaches 21.5 V.

Because the gate drive current sourced from the BOOT capacitor is small, the high-side MOSFET can remain on for many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus, the effective duty cycle of the switching regulator can be high, approaching 100%. The effective duty cycle of the converter during dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low-side diode voltage, and the printed-circuit-board resistance.

Equation 1 calculates the minimum input voltage required to regulate the output voltage and ensure normal operation of the device. This calculation must include tolerance of the component specifications and the variation of these specifications at their maximum operating temperature in the application.

Equation 1.

where

• V_F = Schottky diode forward voltage
• R_dc = DC resistance of inductor and PCB
• R_DS(on) = High-side MOSFET R_DS(on)

At heavy loads, the minimum input voltage must be increased to ensure a monotonic start-up. Equation 2 can be used to calculate the minimum input voltage for this condition.

Equation 2.

where

• D_(max) ≥ 0.9
• IB2SW = 100 µA
• t_SW = 1 / f_SW(MHz)
• VB2SW = VBOOT + V_F
• VBOOT = (1.41 × V_IN – 0.554 – V_F / t_SW – 1.847 × 10³ × IB2SW) / (1.41 + 1 / t_SW)*
• R_DS(on) = 1 / (–0.3 × VB2SW² + 3.577 × VB2SW – 4.246)