Hig h -S p e e d , Dig it a lly Ad ju s t e d
S t e p -Do w n Co n t ro lle rs fo r No t e b o o k CP Us
0/MAX71
+5V
V
BATT
APPROXIMATELY
-0.65V
MAX1710
MAX1711
5Ω
BST
DH
SKIP
1.5mA
V
FORCE
MAX1710
MAX1711
LX
GND
Figure 5. Reducing the Switching-Node Rise Time
Figure 6. Disabling Over/Undervoltage Protection (Test Mode)
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:
es zero with every cycle at maximum load). Inductor
values lower than this grant no further size-reduction
benefit.
1) Input voltage range. The ma ximum va lue
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
) must accommodate the worst-case high
BATT(MAX)
AC adapter voltage. The minimum value (V
)
BATT(MIN)
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.
The ind uc tor rip p le c urre nt a ls o imp a c ts tra ns ie nt-
2) Maximum load current. There are two values to con-
response performance, especially at low V
- V
BATT
OUT
sider. The peak load current (I ) determines
LOAD(MAX)
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 instantaneous component stresses and filtering
re q uire me nts , a nd thus d rive s outp ut c a p a c itor
selection, inductor saturation rating, and the design
of the current-limit circuit. The continuous load cur-
rent (I
) determines the thermal stresses and
LOAD
thus d rive s the s e le c tion of inp ut c a p a c itors ,
MOSFETs, and other critical heat-contributing com-
ponents. Modern notebook CPUs generally exhibit
2
(∆I
)
L
LOAD(MAX)
V
=
SAG
2 C DUTY (V
− V
)
F
BATT(MIN)
OUT
I
= I
· 80%.
LOAD
LOAD(MAX)
3) Switching frequency. This choice determines the
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
In d u c t o r S e le c t io n
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
V
OUT
2
proportional to frequency and VBATT . The optimum
L =
f LIR I
LOAD(MAX)
fre q ue nc y is a ls o a moving ta rg e t, d ue to ra p id
improvements in MOSFET technology that are making
higher frequencies more practical (Table 4).
Example: I
= 7A, V
ripple current or LIR = 0.5.
= 2V, f = 300kHz, 50%
LOAD(MAX)
OUT
4) Inductor operating point. This c hoic e p rovid e s
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-
2V
L =
= 1.9µH (2µH)
300kHz 0.5 7A
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
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
