參數(shù)資料
型號(hào)： MAX1712
廠商： Maxim Integrated Products, Inc.
英文描述： High-Speed, Digitally Adjusted Step-Down Controllers for Notebook CPUs
中文描述： 高速、數(shù)字可調(diào)、降壓型控制器，用于筆記本電腦
文件頁(yè)數(shù)： 18/28頁(yè)
文件大?。?/td> 437K
代理商： MAX1712
M
High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
18
______________________________________________________________________________________
voltage, due to MOSFET switching losses that are
proportional to frequency and VBATT
2
. The optimum
frequency is also a moving target, due to rapid
improvements in MOSFET technology that are making
higher frequencies more practical (Table 5).
4)
Inductor operating point
. This choice provides
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-
es zero with every cycle at maximum load). Inductor
values lower than this grant no further size-reduction
benefit.
The MAX1710/MAX1711/MAX1712s’ 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.
The inductor ripple current also impacts transient-
response performance, especially at low V
BATT
- V
OUT
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:
I
C
DUTY V
F
×
×
2
Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
V
f
LIR
×
×
Example: I
LOAD(MAX)
= 7A, V
OUT
= 2V, f = 300kHz, 50%
ripple current or LIR = 0.5:
V
kHz
×
300
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
is cheap and can work well at 200kHz. The core must be
large enough not to saturate at the peak inductor current
(I
PEAK
):
I
PEAK
= I
LOAD(MAX)
+ (LIR / 2)
I
LOAD(MAX)
Setting the Current Limit
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
LOAD(MAX)
minus half
of the ripple current, therefore:
I
LIMIT(LOW)
> I
LOAD(MAX)
- (LIR / 2)
I
LOAD(MAX)
where I
LIMIT(LOW)
= minimum current-limit threshold volt-
age divided by the R
DS(ON)
of Q2. For the MAX1710, the
minimum current-limit threshold (100mV default setting)
is 90mV. Use the worst-case maximum value for R
DS(ON)
from the MOSFET Q2 data sheet, and add some margin
for the rise in R
DS(ON)
with temperature. A good general
rule is to allow 0.5% additional resistance for each °C of
temperature rise.
Examining the 7A notebook CPU circuit example with a
maximum R
DS(ON)
= 15m
at high temperature reveals
the following:
I
LIMIT(LOW)
= 90mV / 15m
= 6A
6A is greater than the valley current of 5.25A, so the cir-
cuit can easily deliver the full rated 7A using the default
100mV nominal ILIM threshold.
When adjusting the current limit, use a 1% tolerance R
LIM
resistor to prevent a significant increase of errors in the
current-limit tolerance.
Output Capacitor Selection
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 sat-
isfy 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
CORE
converters and other applications where
the output is subject to violent load transients, the output
capacitor’s size 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:
V
I
LOAD MAX
In non-CPU applications, the output capacitor’s size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
R
Vp p
I
LOAD MAX
LIR
ESR
≤
×
-
(
)
R
ESR
DIP
(
≤
)
L
A
=
×
=
μ
μ
2
7
1 9
2
(
)
H
H
L
I
OUT
LOAD MAX
=
(
)
V
L
V
SAG
LOAD MAX
(
BATT MIN
OUT
=
×
(
)
)
(
)
(
)
2
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