MAX1718 Maxim, MAX1718 Datasheet - Page 29

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

iting occurs. The op amp causes current limiting by
lowering the voltage on the ILIM pin. This lowers the
current-limit threshold of the IC’s internal current-limit
circuit, which uses the MOSFET R
op-amp output swing has the ability to adjust the IC’s
internal valley current limit from a value much higher
than ever needed (given the MOSFET’s R
value much lower than required to support a normal
load.
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
circuit responds to the average inductor current rather
than the peak or valley current, eliminating the current
limit’s dependence on inductor ripple current.
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
synchronous rectifier’s R
the op-amp adjustment range is high enough to guar-
antee the required output current. The voltage at the
ILIM pin is given by:
where V
parator. The minimum V
is at the V
calculated when V
The valley current-limit threshold is set at 10% of the
voltage V
mately 10µs time constant at the ILIM input.
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,
I
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.
OUT(MAX)
V
®
ILIM
output swing. Because the bandwidth is low, the
Notebook CPU Step-Down Controller for Intel
=
COMP
ILIM
[
OL
(
✕
R
[
(
12
R8 = V
R
. C13 should be picked to give approxi-
of the comparator. The maximum V
12
is the voltage at the output of the com-
×
R
×
13
______________________________________________________________________________________
R
COMP
FB
14
×
) (
✕
V
+
REF
ILIM
R10 / (R10 + R11). In practice,
RD(ON)
R
= V
12
) (
Mobile Voltage Positioning (IMVP - II)
+
is calculated when V
×
OH
R
R
13
13
in mind to ensure that
at the minimum V
DS(ON)
) (
×
+
R
14
R
13
×
as usual. The
×
V
RD(ON)
COMP
R
14
)
]
ILIM
)
) to a
COMP
]
CC
is
.
An additional benefit of this circuit is that the current-
limit value is proportional to the output voltage setting
(V
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.
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.
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.
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
Figure 16. Improving Current-Limit Accuracy
FB
). When the output voltage setting is lowered, the
MAX1718
CC
supply and analog ground to prevent other
ILIM
REF
DH
FB
Using Skip Mode During Suspend
DL
1nF
R14
100kΩ
R13
20kΩ
30kΩ
R12
V
CC
(SKP/ SDN = V
R10
1.5kΩ
R11
20kΩ
4mΩ
MAX4322
R8
V
1.25V, 19A
OUT
CC
29
)