MAX8745 Maxim Integrated Products, MAX8745 Datasheet - Page 34

• Manufacturer Part Number: MAX8745
• Description: (MAX8744 / MAX8745) Main Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

where V
the charge and discharge paths, respectively. A rea-
sonable minimum value for h is 1.5, while the absolute
minimum input voltage is calculated with h = 1.
The MAX8744/MAX8745 controllers include a minimum
on-time specification, which determines the maximum
input operating voltage that maintains the selected
switching frequency (see the Electrical Characteristics
table). Operation above this maximum input voltage
results in pulse-skipping operation, regardless of the
operating mode selected by SKIP. At the beginning of
each cycle, if the output voltage is still above the feed-
back 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 controller to
effectively operate with a lower switching frequency.
This results in an input threshold voltage at which the
controller begins to skip pulses (V
where f
Careful PC board layout is critical to achieving low
switching losses and clean, stable operation. The
switching power stage requires particular attention
(Figure 9). If possible, mount all the power components
on the top side of the board, with their ground terminals
flush against one another. Follow these guidelines for
good PC board layout:
Keep the high-current paths short, especially at the
ground terminals. This practice is essential for stable,
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
High-Efficiency, Quad Output, Main Power-
Supply Controllers for Notebook Computers
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34
V
IN MIN
______________________________________________________________________________________
(
OSC
CHG
)
=
V
V
is the switching frequency selected by FSEL.
IN SKIP
OUT
and V
(
+
DIS
)
PC Board Layout Guidelines
V
=
CHG
V
are the parasitic voltage drops in
OUT
+
⎛
⎜
⎝
h
f
⎛
⎜
⎝
OSC ON MIN
D
Maximum Input Voltage
MAX
1
t
IN(SKIP)
1
(
−
1
⎞
⎟
⎠
)
(
):
V
⎞
⎟
⎠
OUT
+
V
DIS
)
traces is a difficult task that must be approached in
terms of fractions of centimeters, where a single mil-
liohm of excess trace resistance causes a measurable
efficiency penalty.
Minimize current-sensing errors by connecting CSH_
and CSL_ directly across the current-sense resistor
(R
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 dis-
tance between the inductor and the low-side MOSFET
or between the inductor and the output filter capacitor.
Route high-speed switching nodes (BST_, LX_, DH_,
and DL_) away from sensitive analog areas (REF, FB_,
CSH_, CSL_).
Place the power components first, with ground termi-
nals adjacent (N
anode). If possible, make all these connections on the
top layer with wide, copper-filled areas.
Mount the controller IC adjacent to the low-side MOSFET,
preferably on the back side opposite N
order 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.
Group the gate-drive components (BST_ diode and
capacitor, LDO5 bypass capacitor) together near the
controller IC.
Make the DC-DC controller ground connections as
shown in Figures 1 and 9. This diagram can be viewed
as having two separate ground planes: power ground,
where all the high-power components go, and an ana-
log 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.
Connect the output power planes directly to the output
filter capacitor positive and negative terminals with mul-
tiple vias. Place the entire DC-DC converter circuit as
close to the load as is practical.
SENSE_
).
L
_ source, C
Layout Procedure
IN
, C
OUT
L_
_, and D
and N
H_
L
in
_