Bootstrap circuits are widely used in dc-dc converters to serve as top-side bias voltage supplies. After explaining how bootstrap circuits work, this article will show how versatile they can be. One example bootstrap circuit uses the LTC3728 highside driver. The bias voltage for the high-side driver is produced by the bootstrap supply circuit between the BOOST, INTVCC, and SW pins (Fig. 1).
At the beginning of the initial cycle during startup, the bottom-side MOSFET switch (QBOT) is turned on. This pulls the SW pin to ground, which charges up the bootstrap capacitor (CBST) to INTVCC through diode DBST. At this point, the BOOST pin voltage (VBST) is INTVCC, which is greater than the (hard) turn-on gate-to-source voltage of the top MOSFET switch (QTOP).
Hence, when the high-side driver is enabled during the next half cycle, the driver can turn on QTOP, and the SW pin will rise up to VIN. VBST will then be VIN + INTVCC. Then the cycle repeats itself—QTOP is switched off, QBOT is turned back on, the SW pin is pulled down to ground, and bootstrap capacitor CBST is recharged to INTVCC again. When the switching frequency is sufficiently high, the voltage across the bootstrap capacitor will remain at the level of INTVCC to ensure proper switching of the high-side switch.
In the example application of Figure 1, when QTOP is on, the voltage at the switch pin (VSW) is equal to the input voltage (VIN). At this point, the peak voltage on VBST is the sum of INTVCC and VIN. By adding one more diode and capacitor (DPK and CPK in Fig. 1) to the BOOST pin, a peak detector circuit can provide a dc voltage source equal to VIN + INTVCC.
Figure 2 shows a voltage multiplier circuit based on the basic bootstrap cell and a simple peak detector circuit. Neglecting the voltage drops of the boost diodes (DBSTx) and the peak detector diodes (DPKx), voltage VS2 will be:
VCH + 2 × VS (or, in this case VINTVCC + 2 × VSW)
where 2 is the number of stacked stages of the basic bootstrap cell. In general, a dc voltage of VCH + N × VS can be generated by an N number of stages of the basic bootstrap cell and a peak detector circuit. Considering the LTC3728 application, a voltage of INTVCC + 2 × VIN can be produced using two bootstrap cell stages.
By adding more diodes and capacitors to different voltage sources, designers can generate various dc voltages. To see the possible bootstrap-circuit combinations and resulting dc voltages, click here.