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Capacitor improves efficiency in CPU supply

-April 04, 2002

High efficiency is important for the dc/dc buck converters that supply high currents in notebook PCs. This efficiency extends battery life and minimizes temperature rise. A low-dissipation synchronous rectifier using an external MOSET provides this high efficiency. Synchronous rectifiers require special attention, however. Poor designs allow shoot-through current when the high- and low-side MOSFETs conduct simultaneously. Some designers believe that providing enough dead time between the turn-off of one MOSFET and the turn-on of the other can eliminate this problem, but using dead time is inadequate in some applications. Figure 1 illustrates a step-down power supply in which a step-down controller, the MAX1718, provides the CPU's core supply. Recent CPU cores require a 1 to 2V supply rail at more than 20A of input current. The input-voltage range, on the other hand, is 7 to 20V. This scenario dictates a low duty cycle for the high-side MOSFET.

Obtaining high efficiency with a low duty cycle requires different types of MOSFETs for the high- and low-side devices, Q1 and Q2, respectively. Q1 requires high switching speed even if its on-resistance is relatively high, but Q2 requires low on-resistance even if its switch speed is relatively low. This combination of parameters allows no possibility of shoot-through current when Q2 turns on, because Q1's fast turn-off occurs first. Because Q2's turn-off is slow, however, you must allow enough dead time before Q1 turns on. The MAX1718 solves this problem by monitoring Q2's gate voltage, thereby ensuring that Q1 turns on only after Q2 shuts completely off.

Now, consider a third condition leading to the possibility of shoot-through current: a rise in Q2's gate voltage when Q1 turns on because of high dV/dt at Terminal LX. That condition can appear even with sufficient dead time, because it involves high current flow into Terminal DL through Q2's gate-drain capacitance, QGD. The MAX1718's ample current-sinking ability at DL solves this problem. Sometimes, however, if Q2's gate-drain capacitance is large, the trace from DL is long, or both, you can eliminate the shoot-through current by adding a capacitor of several thousand picofarads between the gate and the source of Q2. Figure 2 shows that the addition of a 4700-pF capacitor improves the high-current efficiency of the circuit in Figure 1 by a considerable margin. However, note that using a too-large gate-source capacitor, QGS, can increase the driver's losses.

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