1.4MHz 1A Synchronous Buck Converter
10
Feedback Divider Resistors
The LX7186A develops a 0.6V reference voltage between the feedback pin, FB, and the signal
ground as shown in Figure 13. The output voltage is set by a resistive divider according to the
following formula:
Keeping the current small (<40µA) in these resistors maximizes efficiency, but making them too small
(<20µA) may allow stray capacitance to cause noise problems and reduce the phase margin of the
error amp loop.
The Output resistor divider values are recommended below.
VOUT
R1
R2
L 2.2µH
SW
FB
COUT
22µF
5 (2)
4 (3)
LX7186A
Figure 13 · Output Circuit
Layout Consideration
PCB layout is very important to the performance of the LX7186A. The traces where switching current
flows should be kept as short as possible. The external components (especially C
IN
) should be
placed as close to the IC as physically possible, therefore use wide and short traces for the main
current paths.
Try to route the feedback trace as far from the inductor and noisy power traces as possible. You
should also make the feedback trace connection as direct as possible and of reasonable thickness.
These two criteria sometimes involve a trade-off, but keeping the trace it away from the inductor and
other noise sources is the more critical of the two. Locate the feedback divider resistor network near
the feedback pin with short leads.
Flood all unused areas on all layers with copper. Flooding with copper will help to reduce the
temperature rise of power components. These copper areas should be connected to one of the input
supplies.
For detailed PCB layout consideration, please refer to LX7186A EVB User Guide.
