參數(shù)資料
型號(hào)： MAX1714
廠商： Maxim Integrated Products, Inc.
英文描述： High-Speed Step-Down Controller for Notebook Computers
中文描述： 高速、降壓型控制器，用于筆記本電腦
文件頁(yè)數(shù)： 16/24頁(yè)
文件大小： 443K
代理商： MAX1714
Inductor Selection
The switching frequency and inductor operating point
determine the inductor value as follows:
V
(V
V
f
LIR
Example: I
LOAD(MAX)
= 8A, V
IN =
7V, V
OUT
= 1.5V,
f = 300kHz, 33% ripple current or LIR = 0.33.
1.5V (7V-1.5V)
7V 300kHz 0.33 8A
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 induc-
tor current (I
PEAK
).
I
PEAK
= I
LOAD(MAX)
+ [(LIR / 2)
·
I
LOAD(MAX)
]
Most inductor manufacturers provide inductors in stan-
dard values, such as 1.0μH, 1.5μH, 2.2μH, 3.3μH, etc.
Also look for nonstandard values, which can provide a
better compromise in LIR across the input voltage range.
For example, Sumida offers 3.1μH and 4.4μH in their
CDRH125 series. If using a swinging inductor (where the
no-load inductance decreases linearly with increasing
current), evaluate the LIR with properly scaled induc-
tance values.
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V
IN
- V
OUT
dif-
ferentials. Low inductor values allow the inductor
current 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 maxi-
mum duty factor, which can be calculated from the on-
time and minimum off-time:
where
and minimum off-time = 400ns typ (see Table 5 for K val-
ues).
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculated
as:
V
2C
where I
PEAK
is the peak inductor current.
Setting the Current Limit
The minimum current-limit threshold must be high
enough to support the maximum load current. The valley
of the inductor current occurs at I
LOAD(MAX)
minus half
of the ripple current (Figure 4); therefore:
I
LIMIT(LOW)
> I
LOAD(MAX)
- (LIR / 2) I
LOAD(MAX)
where I
LIMIT(LOW)
equals minimum current-limit thresh-
old voltage divided by the R
DS(ON)
of Q2. For the
MAX1714, the minimum current-limit threshold using the
100mV default setting is 90mV. Use the worst-case maxi-
mum value for R
DS(ON)
from the MOSFET Q2 data sheet,
and add some margin for the rise in R
DS(ON)
with tem-
perature. A good general rule is to allow 0.5% additional
resistance for each °C of temperature rise.
Examining the 8A circuit example with a maximum
R
DS(ON)
= 12m
at high temperature reveals the follow-
ing:
I
LIMIT(LOW)
= 90mV / 12m
= 7.5A
This 7.5A is greater than the valley current of 6.7A, so the
circuit can easily deliver the full rated 8A using the default
100mV nominal ILIM threshold.
For an adjustable threshold, connect a two-resistor
divider from REF to AGND, with ILIM connected at the
center tap. The external adjustment range of 0.5V to 2.0V
corresponds to a current-limit threshold of 50mV to
200mV. When adjusting the current limit, use 1% toler-
ance resistors to prevent a significant increase of errors in
L I
SOAR
PEAK2
OUT OUT
≈
V
( I
DUTY (V
)
L
2 C
- V
)
SAG
LOAD(MAX)
2
F
IN(MIN)
OUT
=
L
1.49 H
=
=
L =
- V
I
LOAD(MAX)
)
IN
OUT
M
High-Speed Step-Down Controller
for Notebook Computers
16
______________________________________________________________________________________
Good operating point for
compound buck designs
or desktop circuits.
+5V input
600
TON = AGND
450
TON = REF
3-cell Li+ notebook
Useful in 3-cell systems
for lighter loads than the
CPU core or where size is
key.
Considered mainstream
by current standards.
4-cell Li+ notebook
300
TON = Float
200
TON = V
CC
4-cell Li+ notebook efficiency.
COMMENTS
TYPICAL
APPLICATION
FREQUENCY
(kHz)
Table 4. Frequency Selection Guidelines
DUTY
K (V
+ 0.075V) V
+ 0.075V) V
K (V
+ min off-time
OUT
IN
OUT
OUT
=
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