MPQ4572

– 60V, 2A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4572 Rev. 1.0

www.MonolithicPower.com

24

3/9/2020

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Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating HS-FET driver. There are two methods to

charge the bootstrap capacitor (see Figure 3).

The first method is through the main charging

circuit from VCC through a diode. When the HS-

FET is on, VSW is about equal to VIN but exceeds

VCC, and the bootstrap capacitor is not charged.

The best charging period occurs when the LS-

FET is on, and VCC - VSW is at its largest. When

there is no current in the inductor, VSW equals

VOUT, so VCC can only charge BST when VOUT is

very small.

The second method is through the auxiliary

charging circuit from VIN. When the voltage

difference between BST and SW is below the

internal 5V bootstrap regulator, a PMOS pass

transistor (M1) turns on to charge the bootstrap

capacitor. The charging current is much smaller

than that from VCC, but as long as VIN exceeds

VSW, BST can be charged. This function is useful

in sleep mode, when there is not always a switch.

Figure 3: Internal Bootstrap Charging Circuit

Low-Dropout Operation (BST Refresh)

To improve dropout, the MP4572 is designed to

operate at close to 100% duty cycle as long as

the BST-to-SW voltage exceeds 1.4V. When the

BST-to-SW voltage drops below 1.34V, the HS-

FET turns off using a UVLO circuit, which allows

the LS-FET to conduct and refresh the charge on

the BST capacitor. When the input voltage drops,

the HS-FET remains on and close to 100% duty

cycle to maintain output regulation until the BST-

to-SW voltage falls below 1.34V.

Since the supply current sourced from the BST

capacitor is low, the HS-FET can remain on for

more switching cycles than are required to

refresh the capacitor. The means the effective

duty cycle of the switching regulator is high.