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Bootstrap Circuit for Below Ground Operation with Hysteretic Control

IP.com Disclosure Number: IPCOM000174993D
Original Publication Date: 2008-Oct-23
Included in the Prior Art Database: 2008-Oct-23
Document File: 4 page(s) / 149K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

Numerous switch-mode power supply ICs (Integrated Circuit) do not guarantee proper operation, even with low output currents. Failure of the low-side switch causes the switching node VSW of the buck converter to go down to as low as -4V. Traditional bootstrap concepts simply use a diode connected to the internal VC5 supply to charge the bootstrap capacitor (refer to Figure 1). This concept has no precise control of the charge current and can lead to unsafe bootstrap voltage if VSW goes below ground. Some concepts simply clamp the gate of the high-side switch such that it turns-on and pulls up VSW during a loss-of-diode event. The disadvantage of this solution is that the current is then continuously drawn into the clamp while the high-side switch is off. In certain applications an internal low-side switch carries the inductor current during a loss-off-diode condition. During this condition the high-side driver must remain functional as long as the load current is below a certain threshold. This implies that the bootstrap circuit must correctly regulate the capacitor voltage even when VSW is well below ground. The problems with existing bootstrap can be summarized as follows:

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Bootstrap Circuit for Below Ground Operation with Hysteretic Control

Idea: Olivier Trescases, AT-Villach; Derek Bernardon, AT-Villach

Numerous switch-mode power supply ICs (Integrated Circuit) do not guarantee proper operation, even with low output currents. Failure of the low-side switch causes the switching node VSW of the buck converter to go down to as low as -4V. Traditional bootstrap concepts simply use a diode connected to the internal VC5 supply to charge the bootstrap capacitor (refer to Figure 1). This concept has no precise control of the charge current and can lead to unsafe bootstrap voltage if VSW goes below ground. Some concepts simply clamp the gate of the high-side switch such that it turns-on and pulls up VSW during a loss-of-diode event. The disadvantage of this solution is that the current is then continuously drawn into the clamp while the high-side switch is off.

In certain applications an internal low-side switch carries the inductor current during a loss-off-diode condition. During this condition the high-side driver must remain functional as long as the load current is below a certain threshold. This implies that the bootstrap circuit must correctly regulate the capacitor voltage even when VSW is well below ground. The problems with existing bootstrap can be summarized as follows:

• Bootstrap voltage is not well defined.

• Charging current for the bootstrap cap is not well defined.

• Regulation is not maintained for VSW, or current is wasted on clamping when VSW goes several volts below ground.

It is proposed to use a temperature-independent level-shifting scheme to maintain a positive voltage at the inputs of the comparator for large negative voltages on VSW (refer to Figure 2). Thereby the bootstrap capacitor charging current is switched on and off depending on the state of the voltage comparator, and hence the differential value of to ∆VBST. When the high-side switch Ml is on, the entire bootstrap circuit is inactive and the gate-driver draws current from CBST. Once Ml turns off, the switching node is discharged and the bootstrap circuit re-charges CBST through the current source Ichrg.

The switch Sl activates the current source Ichrg when the output of the comparator is low, that is, the bootstrap voltage ∆VBST is too low. The current source Ishift provides an offset at the input of the comparator such that V-=VSW+IShift/R2. The...