MAX1718 Maxim, MAX1718 Datasheet - Page 32

• Manufacturer Part Number: MAX1718
• Description: Notebook CPU Step-Down Controller for Intel Mobile Voltage Positioning IMVP-II
• Manufacturer: Maxim

3) Keep the power traces and load connections short.
4) LX and GND connections to Q2 for current limiting
5) When trade-offs in trace lengths must be made, it’s
6) Ensure the FB connection to the output is short and
7) Route high-speed switching nodes away from sensitive
1) Place the power components first, with ground termi-
Notebook CPU Step-Down Controller for Intel
Mobile Voltage Positioning (IMVP - II)
32
capacitors, the TIME resistor, as well as any other
resistor-dividers.
This is essential for high efficiency. The use of 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 milliohm of excess trace resistance
causes a measurable efficiency penalty.
must be made using Kelvin sense connections to
guarantee the current-limit accuracy. With SO-8
MOSFETs, this is best done by routing power to the
MOSFETs from outside using the top copper layer,
while connecting GND and LX inside (underneath)
the SO-8 package.
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it’s 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 output filter
capacitor.
direct. In voltage-positioned circuits, the FB connection
is at the junction of the inductor and the positioning
resistor.
analog areas (CC, REF, ILIM). Make all pin-strap
control input connections (SKP/SDN, ILIM, etc.) to ana-
log ground or V
nals adjacent (Q2 source, CIN-, COUT-, D1 anode).
______________________________________________________________________________________
CC
rather than power ground or V
Layout Procedure
DD
.
2) Mount the controller IC adjacent to MOSFET Q2,
3) Group the gate-drive components (BST diode and
4) Make the DC-DC controller ground connections as
5) Connect the output power planes (VCORE and system
TRANSISTOR COUNT: 7190
If possible, make all these connections on the top
layer with wide, copper-filled areas.
preferably on the back side opposite Q2 in order to
keep LX-GND current-sense lines and the DL drive line
short and wide. The DL gate trace must be short and
wide, measuring 10 to 20 squares (50mils to 100mils
wide if the MOSFET is 1 inch from the controller IC).
capacitor, V
controller IC.
shown in Figure 19. This diagram can be viewed as
having three separate ground planes: output ground,
where all the high-power components go; the GND
plane, where the GND pin and V
go; and an analog ground plane where sensitive
analog components go. The analog ground plane
and GND plane must meet only at a single point
directly beneath the IC. These two planes are then
connected to the high-power output ground with a
short connection from GND to the source of the low-
side MOSFET Q2 (the middle of the star ground).
This point must also be very close to the output
capacitor ground terminal.
ground planes) directly to the output filter capacitor
positive and negative terminals with multiple vias.
Place the entire DC-DC converter circuit as close to
the CPU as is practical.
DD
bypass capacitor) together near the
Chip Information
DD
bypass capacitors