Notebook CPU Step-Down Controller for Intel
-
Mobile Voltage Positioning (IMVP II)
iting occurs. The op amp causes current limiting by
lowering the voltage on the ILIM pin. This lowers the
R8
current-limit threshold of the IC’s internal current-limit
V
OUT
1.25V, 19A
4mΩ
DH
circuit, which uses the MOSFET R
as usual. The
DS(ON)
op-amp output swing has the ability to adjust the IC’s
MAX1718
internal valley current limit from a value much higher
than ever needed (given the MOSFET’s R
DL
FB
) to a
RD(ON)
value much lower than required to support a normal
load.
R10
1.5kΩ
V
CC
REF
The bandwidth of the ILIM pin is not high, so the speed
of the op amp is not critical. Any op amp or comparator
could be acceptable, as long as its input offset does
not degrade current-limit accuracy excessively, has
input common-mode range to ground and has Rail-to-
Rail® output swing. Because the bandwidth is low, the
circuit responds to the average inductor current rather
than the peak or valley current, eliminating the current
limit’s dependence on inductor ripple current.
R14
100kΩ
R12
30kΩ
ILIM
1nF
MAX4322
R13
20kΩ
R11
20kΩ
Similar to a foldback current limit, this circuit must be
carefully designed to guarantee startup. The op amp
must be incapable of setting the current limit to zero or
else the power supply may be unable to start. The
three-way divider from REF to ground to the op-amp
output allows the op amp to vary the MAX1718’s inter-
nal current-limit threshold from 21mV (severely limiting
current) to 182mV (more than guaranteeing the maxi-
mum required output current). These divider resistors
should be chosen with the required current and the
Figure 16. Improving Current-Limit Accuracy
An additional benefit of this circuit is that the current-
limit value is proportional to the output voltage setting
(V ). When the output voltage setting is lowered, the
FB
current limit automatically adjusts to a more appropriate
level, providing additional protection without compro-
mising performance since the reduction of the required
load current is greater than that of the output voltage
setting. In some cases, the current required to slew the
output capacitor may be large enough to require the
current limit to be increased beyond what is necessary
to support the load.
synchronous rectifier’s R
in mind to ensure that
RD(ON)
the op-amp adjustment range is high enough to guar-
antee the required output current. The voltage at the
ILIM pin is given by:
This circuit is completely compatible with the circuit of
Figure 15. If the two circuits are used together, the
MAX4326 dual op amp in a µMAX package can
replace the two single devices, saving space and cost.
If both are used, the reduced R8 value makes the op-
amp input offset more significant. Additional margin
might be needed, depending on the magnitude of R8’s
reduction.
(R12 ×R13 × V
)+(R13 ×R14 × V
)
[
]
REF
COMP
V
=
ILIM
(R12 ×R14)+(R12 ×R13)+(R13 × R14)
[
]
where V
is the voltage at the output of the com-
COMP
parator. The minimum V
is calculated when V
COMP
ILIM
is at the V
of the comparator. The maximum V
is
.
CC
OL
ILIM
calculated when V
= V
at the minimum V
COMP
OH
The valley current-limit threshold is set at 10% of the
Although the op amp should be placed near R8 to mini-
mize input noise pickup, power it from the MAX1718’s
voltage V
. C13 should be picked to give approxi-
ILIM
mately 10µs time constant at the ILIM input.
quiet V
supply and analog ground to prevent other
CC
noise problems.
The actual threshold at which the op amp begins to
limit current is determined by R8 and the R10/R11
divider values and is very easy to set. Ideally,
Using Skip Mode During Suspend
(SKP/SDN = V )
CC
✕
✕
I
R8 = V
R10 / (R10 + R11). In practice,
OUT(MAX)
FB
Typically, for the MAX1718’s intended application, the
minimum output currents are too high to benefit from
pulse-skipping operation in all active CPU modes.
Furthermore, Skip mode can be a hindrance to properly
executing downward output voltage transitions (see the
some margin must be added for resistor accuracy, op-
amp input offset, and general safety. With the op amp
shown and 1% resistors, 10% margin is adequate.
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
