MAX1717 Maxim, MAX1717 Datasheet - Page 25

• Manufacturer Part Number: MAX1717
• Description: Dynamically Adjustable / Synchronous Step-Down Controller for Notebook CPUs
• Manufacturer: Maxim

where t
Characteristics) and K is from Table 3.
The switching frequency and operating point (% ripple or
LIR) determine the inductor value as follows:
Example: I
ƒ
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
MAX1717, the minimum current-limit threshold (100mV
default setting) is 90mV. Use the worst-case maximum
value for R
add some margin for the rise in R
ture. A good general rule is to allow 0.5% additional
resistance for each °C of temperature rise.
SW
V
SAG
= 300kHz, 30% ripple current or LIR = 0.30.
I
LIMIT(LOW)
I
=
PEAK
OFF(MIN)
LIMIT(LOW)
2
(
L
L
⋅
I
DS(ON)
LOAD(MAX)
LOAD
C
=
=
= I
OUT
7
V
V
LOAD(MAX)
IN
> I
1
Step-Down Controller for Notebook CPUs
is the minimum off-time (see Electrical
⋅
−
PEAK
⋅
1 6 7
from the MOSFET Q2 data sheet, and
______________________________________________________________________________________
300
⋅ ƒ
LOAD(MAX)
V
.
I
V
LOAD
OUT
equals the minimum current-limit
OUT IN
V V
SW
kHz
(
= 14A, V
).
Setting the Current Limit
Dynamically Adjustable, Synchronous
(




⋅
2
V
K
LIR I
)
⋅
−
+ (LIR / 2) I
2
0 30 14



1 6
.
⋅
V
.
−
L K
⋅
- (LIR / 2) I
IN
V
Inductor Selection
V



LOAD MAX
IN
⋅
)
OUT
−
V
LOAD(MAX)
IN
DS(ON)
DS(ON)
V
V
= 7V, V
A
OUT
V
OUT
IN
(
)
=
LOAD(MAX)
0 98

 −

LOAD(MAX)
+
.
)
of Q2. For the
with tempera-
t
OUT
OFF MIN
t
µ
OFF MIN
minus half
H
(
= 1.6V,
(
)



)




Examining the Figure 1 example with a Q
R
+100°C reveals the following:
and the required valley current limit is:
Therefore, the circuit can deliver the full-rated 14A
using the default ILIM threshold.
When delivering 14A of output current, the worst-case
power dissipation of Q2 is 1.48W. With a thermal resis-
tance of 60°C/W and each MOSFET dissipating 0.74W,
the temperature rise of the MOSFETs is 60°C/W · 0.74W
= 44.5°C, and the maximum ambient temperature is
+100°C - 44.5°C = +55.5°C. To operate at a higher
ambient temperature, choose lower R
or reduce the thermal resistance. You could also raise
the current-limit threshold, allowing operation with a
higher MOSFET 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
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
= 5.5mΩ at T
I
LIMIT(LOW)
R
converters and other applications where
ESR
CC
> 14A - (0.3012) 14A = 11.9A
≤ V
Output Capacitor Selection
= 90mV / 7.5mΩ = 11.9A
for a default 100mV current-limit
STEP
J
= +25°C and 7.5mΩ at T
/ I
LOAD(MAX)
DS(ON)
2
MOSFETs
maximum
J
25
=