This section describes how to best
design a buck converter to support 6 V input supply, and step it down to 1.2 V
output and support up to 3 A. The same steps can be taken to design it for other
design requirements.
Selection of components:
Inductor: The 6
VIN to 1.2 V output voltage allows the use of 0.68 µH inductor
with reasonable inductor current ripple if 2.5 MHz is chosen. A small
size 2.0 mm × 1.6 mm 0.68 µH inductor can be used for this such as the
“DFE201612E-R68M#” from Murata.
Input capacitor: Input
voltage requirement for this design is only 6 V. Therefore, 10 µF with only 10 V
rating can be used.
Output capacitor: The
recommended output capacitor in the data sheet is 22 µF. In this example, the
output is set to 1.2 V, therefore only 6 V voltage rating can be used.
Feedback: 1.2 V
VOUT is one of the 16 options VSET can support, to save area and
achieve better accuracy, the internal voltage divider can be used in this
example. Internal feedback should give a total system accuracy of ±1.25% versus
1.9% if external feedback is used.
Mode and Smart
Configuration: A low VOUT can tolerate low switching
frequency with good inductor current ripple. Both 1 MHz or 2.5 MHz can be
chosen. However, the goal is to have the smallest solution size possible, and
2.5 MHz allows the use of a 0.68 µH inductor with acceptable ripple. Power save
mode is preferred to provide high efficiency at light loads. Thus, 26.1 kΩ is
connected on the Mode/S-CONF pin to GND.
EN, Soft start, and PG: If
there is no need for soft start capacitor and Power Good features, these pins
can be left floating. The EN pin can be connected directly to VIN. These add
some additional BOM savings.
Here is a suggested schematic for this
example:
Figure 1-1 Design Example Schematic
Table 1-1 Components PCB Area
Component
Size and Rating
Area
CIN
10 µF, 10 V, 0805, X7R
2.5 mm2
COUT
22 µF, 6 V, 0805, X7R
2.5 mm2
Inductor
0.68 µH, 2.0 mm × 1.6 mm × 1.2 mm, 33 mΩ
3.2 mm2
Mode/S-CONF resistor
26.1 kΩ, ±1%, 0402
0.5 mm2
TPS62903
Buck Converter, 1.5 mm × 2.0 mm
3.0 mm2
Routing
Estimated Routing Area
13.3 mm2
Total Area
Routing plus components
25 mm2
Figure 1-2 provides a
layout example. CIN, COUT, and the inductor L are placed as close as possible to the
pin of the device. The SW node trace is kept small for better noise performance.
Vias are added on GND, VIN, and VOUT traces to help improve thermal dissipation of
the board.