MAX1718 Maxim, MAX1718 Datasheet - Page 25

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

where t
Electrical Characteristics tables) and K is from Table 2.
The switching frequency and operating point (% ripple or
LIR) determine the inductor value as follows:
Example: I
f
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The valley
of the inductor current occurs at I
of the ripple current; therefore:
where I
threshold voltage divided by the R
MAX1718 Figure 1 circuit, the minimum current-limit
threshold with V
worst-case maximum value for R
FET Q2 data sheet, and add some margin for the rise in
R
allow 0.5% additional resistance for each °C of temper-
ature rise.
SW
V
DS(ON)
SAG
step. The amount of output sag is also a function of
the maximum duty factor, which can be calculated
from the on-time and minimum off-time:
= 300kHz, 30% ripple current or LIR = 0.30.
I
LIMIT(LOW)
I
Notebook CPU Step-Down Controller for Intel
=
L
PEAK
LIMIT(LOW)
OFF(MIN)
2
L
with temperature. A good general rule is to
=
(
×
I
LOAD(MAX)
=
LOAD
C
= I
7
V
OUT
IN
V
LOAD(MAX)
> I
×
1
ILIM
1 25 7
−
×
PEAK
.
300
______________________________________________________________________________________
×
LOAD(MAX)
is the minimum off-time (see the
I
LOAD
equals the minimum current-limit
V
V
f
V V
= 105mV is about 95mV. Use the
OUT
kHz
SW
OUT
= 19A, V
(
).
Setting the Current Limit
2
×
)




−
×
(
+ (LIR / 2) I
K
2
V
0 30 19
1 25
IN
Mobile Voltage Positioning (IMVP - II)
×



.
.
LIR
- (LIR / 2) I
V
L K
IN
IN
Inductor Selection
−



V
×
DS(ON)
)
LOAD(MAX)
−
×
= 7V, V
DS(ON)
V
V
V
IN
V
OUT
V
OUT
A
OUT
I
IN
LOAD MAX
LOAD(MAX)
=
)
LOAD(MAX)
from the MOS-
0 60
+

 −

of Q2. For the
OUT
.
(
t
OFF MIN
t
µ
minus half
OFF MIN
H
= 1.25V,
(
)
(
)



)




Examining the Figure 1 example with a Q2 maximum
R
+125°C reveals the following:
and the required valley current limit is:
Since 16.7A is greater than the required 16.2A, the cir-
cuit can deliver the full-rated 19A.
When delivering 19A of output current, the worst-case
power dissipation of Q2 is 1.95W. With a thermal resis-
tance of 60°C/W and each MOSFET dissipating 0.98W,
the temperature rise of the MOSFETs is 60°C/W
0.98W = 58°C, and the maximum ambient temperature
is +125°C - 58°C = +67°C. To operate at a higher
ambient temperature, choose lower R
or reduce the thermal resistance. Raising the current-
limit threshold allows for operation with a higher MOS-
FET junction temperature.
Connect ILIM to V
threshold. For an adjustable threshold, connect a resistor
divider from REF to GND, with ILIM connected to the
center tap. The external adjustment range of 0.5V to 3.0V
corresponds to a current-limit threshold of 50mV to
300mV. When adjusting the current limit, use 1% toler-
ance resistors and a 10µA divider current to prevent a
significant increase of errors in the current-limit toler-
ance.
The output filter capacitor must have low enough effective
series resistance (ESR) to meet output ripple and load-
transient requirements, yet have high enough ESR to
satisfy stability requirements. Also, the capacitance
value must be high enough to absorb the inductor energy
going from a full-load to no-load condition without tripping
the OVP circuit.
In CPU V
the output is subject to violent load transients, the output
capacitor’s size typically depends on how much ESR is
needed to prevent the output from dipping too low
under a load transient. Ignoring the sag due to finite
capacitance:
The actual microfarad capacitance value required often
relates to the physical size needed to achieve low ESR,
as well as to the chemistry of the capacitor technology.
Thus, the capacitor is usually selected by ESR and volt-
DS(ON)
I
LIMIT(LOW)
CORE
= 3.8mΩ at T
I
LIMIT(LOW)
R
converters and other applications where
ESR
> 19A - (0.30 / 2) 19A = 16.2A
CC
≤ V
Output Capacitor Selection
= 95mV / 5.7mΩ = 16.7A
for a default 100mV current-limit
STEP
J
= +25°C and 5.7mΩ at T
/ I
LOAD(MAX)
DS(ON)
MOSFETs
J
25
=
✕