Texas Instruments SLVU013 - Manual (Page 10)

Synchronous Buck Regulator Operation
1-2
1.1 Synchronous Buck Regulator Operation
The synchronous buck converter is a variation of the traditional buck
converter. The main switching device is usually a power MOSFET and is
driven in the same manner as in a traditional buck converter. The freewheeling
rectifier, usually a Schottky device, is replaced by a power MOSFET and is
driven in a complementary or synchronous fashion relative to the main
switching device; when one MOSFET is on, the other is off. The freewheeling
MOSFET is selected so that its ON voltage drop is less than the forward drop
of the original freewheeling rectifier, thus increasing conversion efficiency. A
very important design issue when using a synchronous buck converter is
preventing cross-conduction of the two power MOSFETs, i.e., preventing both
MOSFETs from being on simultaneously. A small amount of deadtime is
necessary.
Figure 1 shows a simplified schematic of a synchronous buck converter. The
TPS56xx senses the output voltage and then drives Q1 and Q2 depending on
the sensed voltage. The TPS56xx senses the voltage at the junction of Q1, Q2,
and L1 and uses it to actively prevent simultaneous conduction of Q1 and Q2.
Figure 1–1. Simplified Synchronous Buck Converter Schematic
TPS56xx
Q1
Q2
+
+
VI
VO
L1
C