Circuit secures bootstrap operation under light load

-May 12, 2011

Circuit secures bootstrap operation under light load figure 2A previous Design Idea highlights a potential issue of a bootstrap IC under light-load or prechargedload operation (Reference 1). The circuit in Figure 1, with the additional circuit in the green box, fixes the problem of a voltage dip in the bus-voltage signal. The waveforms in Figure 2 demonstrate how this problem takes place in buck converter IC1B when its output voltage, VBUS, dips below the regulation point at a fixed rate under no load. By inspecting the other traces in Figure 2, you can conclude that the dip of VBUS occurs when the bootstrap voltage falls below its threshold of 8.66V (Trace 3), causing the buck converter’s switching action to cease. This situation intensifies when the bus voltage approaches the input voltage.

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Design Ideas
During freewheeling operation of DCM (discontinuous-conduction mode), the output signal (Figure 3, Trace 4) tends to settle at the bus voltage after inductor L1 completely discharges. This action prevents bootstrap capacitor CBS from charging, which eventually causes the bootstrap voltage in Figure 2 to fall below 8.66V. Hence, the buck converter stops switching.

Circuit secures bootstrap operation under light load figure 3The circuit in Figure 1’s green box aims to solve the problem. It starts by tapping the input signal to the bootstrap’s high-side driver to generate an inverted and delayed short pulse to control Q2. Upon activation, Q2 forces the output signal momentarily low, which provides an opportunity for CBS to charge. R8, R9, R10, R11, and C9 set the turn-on period of Q2. This period must not exceed the dead time of the PWM (pulse-width-modulated) signal. If Q2’s turn-on time is too long, the converter’s efficiency will degrade, or the CBS might not sufficiently charge. Inadequate charging of Q2 involves multiple component values and operating parameters, such as Q2’s turn-on time, and you might have to empirically tune the delay time to accommodate for these effects. The values in Figure 1 produce a Q2-turn-on time of 1 μsec and delay time of 450 nsec in a 70-kHz switching frequency.

Circuit secures bootstrap operation under light load figure 4The Q5 network is optional. It lets you disable operation of Q2 when it is not necessary by linking the P_on/off signal to an open collector, Q6. The low section of IC1A drives Q2. You must experimentally select the value of R6. A resistance value that is too low induces larger current spikes upon activation of Q2. On the other hand, a resistance value that is too high causes CBS to insufficiently charge.

Resistor R7 and capacitor C8 control the delay time between the falling edge of the input signal to the bootstrap’s high-side driver and the rising edge of IC1A’s LVG (low-voltage) pulse. Figure 4 displays the waveform of the same converter after the inclusion of the additional circuit. In this case, VBUS (Trace 1) remains stable and the output signal from the buck regulator switches continuously, lacking the gaps with switching that the waveforms of Figure 2 show.



Reference
  1. Larson, Justin, and Frank Kolanko, “Buck regulator handles light loads,” EDN, Sept 9, 2010, pg 48.

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