The literal meaning of bootstrap circuit is a circuit that lifts itself up. In this section, we introduce the boost circuit that uses a capacitor, which is a common circuit in electronic circuits. We often see bootstrap capacitors in peripheral devices of ICs, such as CBOOT in the synchronous buck converter (BUCK) circuit in Figure 1-44. If the capacitor connected in parallel with the power input or output is disconnected, at least the power can still output a target voltage (with poor stability and noise performance), but if the CBOOT capacitor is abnormal, the power will not work at all.
Figure 1-44 Bootstrap capacitor CBOOT in a switching power supply
Why do we need a bootstrap circuit? This is because in some circuits, a bridge circuit is built using MOSFETs, as shown in Figure 1-45. The condition for the NMOSFET to turn on is easy to achieve, with the gate-to-source voltage VGS of the lower transistor Q2 exceeding VGS(th), which is usually low and therefore easy to achieve. However, for the upper transistor Q1, the source already has a certain voltage. If we want to directly drive the gate to satisfy the condition VGS>VGS(th), the gate voltage needs to be higher than the source voltage, which is difficult to achieve (MOSFET-related information is discussed in Introduction to MOSFET Characteristics).
Thus, the bootstrap circuit was born.
Figure 1-45 Synchronous switching power supply topology with two MOSFETs
With the bootstrap circuit, a high voltage can be easily generated on the gate of the upper transistor to drive the upper MOSFET. The specific principle is as follows:
As shown in Figure 1-46, the input total voltage VIN is output as a DC voltage V through an internal regulator, which is usually an LDO structure power supply (LDO principle is discussed in detail in Chapter 2), and is used to charge CBOOT (C1). When the lower transistor Q2 turns on, the SW voltage is 0, and the charging loop for the CBOOT capacitor is completed through the internal regulator output voltage V, diode, C1, Q2, and ground, and the voltage at both ends of the capacitor is approximately equal to V.
Figure 1-46 CBOOT charging path
When the lower transistor Q2 turns off, the discharge path of the capacitor is shown in Figure 1-47. The voltage at point A is now higher than the SW position voltage by V, which means that the gate-to-source voltage of Q1 is higher than VGS(th), so the upper transistor Q1 can be turned on, and the voltage at point A becomes V+Vsw, achieving voltage boost. The capacitor lifts itself up.
Figure 1-47 CBOOT discharge path
Figure 1-48 shows the measured waveform of the CBOOT voltage. The yellow and green curves are the voltage waveforms at both ends of the capacitor relative to the system GND, and the pink curve is the difference between the green and yellow curves, which is the voltage waveform of both ends of the capacitor. As can be seen, the voltage at both ends of the capacitor remains unchanged with the switching of the transistor, remaining at the voltage of the internal LDO. However, the voltage at both ends of the capacitor relative to the system GND keeps fluctuating, sometimes rising and sometimes falling. This can make the high-side voltage of the capacitor high enough to drive the upper transistor, which is consistent with the analysis process above.
Figure 1-48 Measured waveform at both ends of CBOOT.
The above is the basic principle of the
bootstrap capacitor.
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