Rainbow Electronics MAX8775 - Manual (Page 28)

MAX8775
where a single mΩ of excess trace resistance caus-
es a measurable efficiency penalty.
• Minimize current-sensing errors by connecting
CSH_ and CSL_ directly across the current-sense
resistor (R
SENSE_ ).
• When trade-offs in trace lengths must be made, it is
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it is better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and the low-
side MOSFET or between the inductor and the out-
put filter capacitor.
• Route high-speed switching nodes (BST_, LX_, DH_,
and DL_) away from sensitive analog areas (REF,
REFIN_, CSH_, CSL_).
Layout Procedure
1) Place the power components first, with ground ter-
minals adjacent (N L _ source, C IN , C OUT _, and D L _
anode). If possible, make all these connections on
the top layer with wide, copper-filled areas.
2) Mount the controller IC adjacent to the low-side
MOSFET, preferably on the back side opposite N L_
and N H_ to keep LX_, GND, DH_, and the DL_ gate-
drive lines short and wide. The DL_ and DH_ gate
traces must be short and wide (50 mils to 100 mils
wide if the MOSFET is 1in from the controller IC) to
keep the driver impedance low and for proper
adaptive dead-time sensing.
3) Group the gate-drive components (BST_ capacitor,
V
DD
bypass capacitor) together near the controller IC.
4) Make the DC-DC controller ground connections as
shown in Figures 1, 2, and 11. This diagram can be
viewed as having two separate ground planes:
power ground, where all the high-power compo-
nents go; and an analog ground plane for sensitive
analog components. The analog ground plane and
power ground plane must meet only at a single
point directly at the IC.
5) Connect the output power planes directly to the out-
put filter capacitor positive and negative terminals
with multiple vias. Place the entire DC-DC converter
circuit as close to the load as is practical.
Dual and Combinable Graphics Core
Controller for Notebook Computers
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