MAX1711 Maxim, MAX1711 Datasheet - Page 17

• Manufacturer Part Number: MAX1711
• Description: High-Speed / Digitally Adjusted Step-Down Controllers for Notebook CPUs
• Manufacturer: Maxim

ry design trade-off lies in choosing a good switching fre-
quency and inductor operating point, and the following
four factors dictate the rest of the design:
1) Input voltage range. The maximum value
2) Maximum load current. There are two values to con-
3) Switching frequency. This choice determines the
4) Inductor operating point. This choice provides
Figure 5. Reducing the Switching-Node Rise Time
(V
AC adapter voltage. The minimum value (V
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery selector
switches. If there is a choice at all, lower input volt-
ages result in better efficiency.
sider. The peak load current (I
the instantaneous component stresses and filtering
requirements, and thus drives output capacitor
selection, inductor saturation rating, and the design
of the current-limit circuit. The continuous load cur-
rent (I
thus drives the selection of input capacitors,
MOSFETs, and other critical heat-contributing com-
ponents. Modern notebook CPUs generally exhibit
I
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage, due to MOSFET switching losses that are
proportional to frequency and VBATT
frequency is also a moving target, due to rapid
improvements in MOSFET technology that are making
higher frequencies more practical (Table 4).
trade-offs between size vs. efficiency. Low inductor
values cause large ripple currents, resulting in the
smallest size, but poor efficiency and high output
noise. The minimum practical inductor value is one
that causes the circuit to operate at the edge of criti-
cal conduction (where the inductor current just touch-
LOAD
BATT(MAX)
LOAD
= I
MAX1710
MAX1711
LOAD(MAX)
Step-Down Controllers for Notebook CPUs
) must accommodate the worst-case high
) determines the thermal stresses and
______________________________________________________________________________________
BST
DH
· 80%.
LX
5
LOAD(MAX)
+5V
2
. The optimum
V
High-Speed, Digitally Adjusted
BATT
) determines
BATT(MIN)
)
The inductor ripple current also impacts transient-
response performance, especially at low V
differentials. Low inductor values allow the inductor cur-
rent to slew faster, replenishing charge removed from the
output filter capacitors by a sudden load 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:
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
Example: I
ripple current or LIR = 0.5.
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
Figure 6. Disabling Over/Undervoltage Protection (Test Mode)
es zero with every cycle at maximum load). Inductor
values lower than this grant no further size-reduction
benefit.
The MAX1710/MAX1711’s pulse-skipping algorithm
initiates skip mode at the critical-conduction point. So,
the inductor operating point also determines the load-
current value at which PFM/PWM switchover occurs.
The optimum point is usually found between 20% and
50% ripple current.
V
SAG
LOAD(MAX)
L
2
C DUTY V
L
300
F
MAX1710
MAX1711
f LIR I
kHz
(
= 7A, V
I
2
LOAD MAX
V
GND
0 5 7
(
V
.
OUT
LOAD MAX
BATT MIN
(
OUT
SKIP
A
(
(
Inductor Selection
)
= 2V, f = 300kHz, 50%
)
2
APPROXIMATELY
1 9
)
1.5mA
L
.
)
-0.65V
V
H
OUT
(
2
BATT
)
V
H
FORCE
)
- V
OUT
17