MAX1533A Datasheet, PDF (34/38 Pages) Maxim Integrated Products – High-Efficiency, 5x Output, Main Power-Supply Controllers for Notebook Computers

High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
above the feedback-threshold voltage, the controller
does not trigger an on-time pulse, effectively skipping a
cycle. This allows the controller to maintain regulation
above the maximum input voltage, but forces the con-
troller to effectively operate with a lower switching fre-
quency. This results in an input threshold voltage at
which the controller begins to skip pulses (V_IN(SKIP)):
⎛
V_IN(SKIP)
=
V_OUT
⎜
⎝
f_OSC
1
⎞
t_ON(MIN)
⎟
⎠
where f_OSC is the switching frequency selected by FSEL.
PC Board Layout Guidelines
Careful PC board layout is critical to achieving low
switching losses and clean, stable operation. The
switching power stage requires particular attention
(Figure 11). If possible, mount all of the power compo-
nents on the top side of the board, with their ground
terminals flush against one another. Follow these guide-
lines for good PC board layout:
• Keep the high-current paths short, especially at the
ground terminals. This practice is essential for sta-
ble, jitter-free operation.
• Keep the power traces and load connections short.
This practice is essential for high efficiency. Using
thick copper PC boards (2oz vs. 1oz) can enhance
full-load efficiency by 1% or more. Correctly routing
PC board traces is a difficult task that must be
approached in terms of fractions of centimeters,
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,
FB_, CSH_, CSL_).
Layout Procedure
1) Place the power components first, with ground termi-
nals 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 to 100 mils wide if
the MOSFET is 1 inch from the controller IC) to keep
the driver impedance low and for proper adaptive
dead-time sensing.
3) Group the gate-drive components (BST_ diode and
capacitor, LDO5 bypass capacitor) together near
the controller IC.
4) Make the DC-DC controller ground connections as
shown in Figures 1 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.
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