Rainbow Electronics MAX8742 - Manual (Page 27)

MAX8741/
M
AX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
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d) LX node (MOSFETs, rectifier cathode, induc
tor): 15mm max length
Ideally, surface-mount power components are butted up
to one another with their ground terminals almost touch-
ing. These high-current grounds are then connected to
each other with a wide filled zone of top-layer copper so
they do not go through vias. The resulting top layer “sub-
ground-plane” is connected to the normal inner-layer
ground plane at the output ground terminals, which
ensures that the IC’s analog ground is sensing at the sup-
ply’s output terminals without interference from IR drops
and ground noise. Other high-current paths should also
be minimized, but focusing primarily on short ground and
current-sense connections eliminates about 90% of all PC
board layout problems.
2) Place the IC and signal components. Keep the main
switching nodes (LX nodes) away from sensitive
analog components (current-sense traces and REF
capacitor). Place the IC and analog components on
the opposite side of the board from the power-
switching node.
Important: The IC must be no more
than 10mm from the current-sense resistors. Keep
the gate-drive traces (DH_, DL_, and BST_) shorter
than 20mm and route them away from CSH_, CSL_,
and REF.
3) Use a single-point star ground where the input
ground trace, power ground (subground plane), and
normal ground plane meet at the supply’s output
ground terminal. Connect both IC ground pins and all
IC bypass capacitors to the normal ground plane.
MAX8741/MAX8742
SENSE RESISTOR
HIGH-CURRENT PATH
Figure 9. Kelvin Connections for the Current-Sense Resistors