MAX1718 Maxim, MAX1718 Datasheet - Page 28

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

Notebook CPU Step-Down Controller for Intel
Mobile Voltage Positioning (IMVP - II)
application circuit (Figure 1) voltage positioning is
accomplished using a droop resistor (R8), which can
dissipate over 1W. Although the power savings in the
processor is much greater than the dissipation in the
resistor, 1W of dissipation is still far from ideal.
The resistor is a necessary component because accu-
rate voltage positioning depends on an accurate cur-
rent-sense element. But it is not necessary to drop the
entire positioning voltage across this resistor. The cir-
cuit of Figure 15 uses an external op amp to add gain
to R8’s voltage signal, allowing the resistor value and
power dissipation to be divided by the gain factor.
The recommended range for the gain is up to about 4,
with preferred practical values around 1.5-3. There are
several difficulties with high gains. If high gain is used,
the sense-resistor value will be very small (<1mΩ). The
sense signal will also be small, potentially causing
noise and stability problems. Also, output voltage and
positioning accuracy are essential. A smaller sense sig-
nal will reduce accuracy, as will any op amp input volt-
age offset, which is increased by the gain factor.
The op amp output directly drives FB. To ensure stabili-
ty, the output voltage ripple and the ripple signal
across R8 must be delivered with good fidelity. To pre-
serve higher harmonics in the ripple signal, the circuit
bandwidth should be about 10 times the switching fre-
quency.
In addition to lowering power dissipation, the gain
stage provides another benefit; it eases the task of pro-
viding the required positioning slope, using available
discrete values for R8. A lower value resistor can be
used and the gain adjusted to deliver the desired
slope. Sometimes the desired slope is not well known
before the final PC board is evaluated. A good practice
is to adjust the final gain to deliver the correct voltage
slope at the processor pins, adjusting for the actual
copper losses in the supply and ground paths. This
does not remove the requirement to minimize copper
losses because they vary with temperature and PC
board production lot. But it does provide an easy, prac-
tical way to account for their typical expected voltage
drops.
Replacing the droop resistor with a lower value resistor
and a gain stage does not affect the MAX1718 stability
criteria. The IC cannot distinguish one from the other,
as long as the required signal integrity is maintained.
R8’s effective value can be used to guarantee stability
with extremely low-ESR (ceramic) output capacitors
(see the Output Capacitor Stability Considerations sec-
tion). The effective value is the resistor value that would
28
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result in the same signal delivered to the MAX1718’s FB
pin, or R8 times the op-amp circuit’s gain.
Although the op amp should be placed near R8 to mini-
mize input noise pickup, power it from the MAX1718’s
quiet V
noise problems.
The MAX1718’s integrated current limit uses the syn-
chronous rectifier’s R
ment. This dependence on a poorly specified
resistance with high temperature variation means that
the integrated current limit is useful mainly in high-over-
load and short-circuit conditions. A moderate overload
may be tolerated indefinitely. This arrangement is toler-
able because there are other ways to detect overload
conditions and take appropriate action. For example, if
the CPU draws excessive current (but not enough to
activate the current limit) the CPU will heat up, and
eventually the system will take notice and shut down.
While this approach is usually acceptable, it is far from
optimal. An inaccurate current limit causes component
specification difficulties. What values should be used
for inductor saturation ratings, MOSFET peak current
requirements, and power dissipation requirements? An
accurate current limit makes these issues more man-
ageable. The circuit of Figure 16 uses an external op
amp, together with the voltage-positioning resistor (R8)
to implement an accurate inductor current limit.
A voltage divider from the positive side of R8 creates a
threshold several mVs below the output. When the volt-
age drop across R8 exceeds the threshold, current lim-
Figure 15. Lowering Voltage-Positioning Power Dissipation
CC
MAX1718
supply and analog ground to prevent other
DH
DL
FB
High-Accuracy Current Limit
DS(ON)
A = 2
1kΩ
R5
V
CC
for its current-sense ele-
MAX4322
510Ω
1kΩ
R6
R8
V
1.25V, 19A
OUT