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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 |
| 文件頁數(shù)： | 15/24頁 |
| 文件大?。?/td> | 443K |
| 代理商： | MAX1714 |
M
High-Speed Step-Down Controller
for Notebook Computers
______________________________________________________________________________________
15
2)
Maximum load current
. There are two values to con-
sider. The
peak load current
(I
LOAD(MAX)
) determines
the instantaneous component stresses and filtering
requirements, and thus drives output capacitor selec-
tion, inductor saturation rating, and the design of the
current-limit circuit. The
continuous load current
(I
LOAD
) determines the thermal stresses and thus dri-
ves the selection of input capacitors, MOSFETs, and
other critical heat-contributing components. Modern
notebook CPUs generally exhibit:
I
LOAD
= I
LOAD(MAX)
·
80%
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
proportional to frequency and V
IN2
. The optimum fre-
quency is also a moving target, due to rapid improve-
ments in MOSFET technology that are making higher
frequencies more practical (Table 4).
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 rip-
ple. The minimum practical inductor value is one that
causes the circuit to operate at the edge of critical
conduction (where the inductor current just touches
zero with every cycle at maximum load). Inductor val-
ues lower than this grant no further size-reduction
benefit.
The MAX1714’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.
These four factors impact the component selection
process. Selecting components and calculating their
effect on the MAX1714’s operation is best done with a
spreadsheet. Using the formulas provided, calculate the
LIR (the ratio of the inductor ripple current to the
designed maximum load current) for both the minimum
and maximum input voltages. Maintaining an LIR within a
20% to 50% range is prudent. The use of a spreadsheet
allows quick evaluation of component selection.
Table 3. Operating Mode Truth Table
SHDN
SKIP
DL
MODE
COMMENTS
0
X
High
Shutdown
Low-power shutdown state. DL is forced to V
DD
, enforcing OVP. I
CC
< 1μA typ.
1
Below
AGND
Switching
No Fault
Test mode with OVP, UVP, and thermal faults disabled and latches cleared. Otherwise
normal operation, with automatic PWM/PFM switchover for pulse skipping at light loads
(Figure 6).
1
V
CC
Switching
Run (PWM),
Low Noise
Low-noise operation with no automatic switchover. Fixed-frequency PWM action is
forced regardless of load. Inductor current reverses at light load levels. Low noise,
high I
Q
.
Normal operation with automatic PWM/PFM switchover for pulse skipping at light loads.
Best light-load efficiency.
Fault latch has been set by OVP, output UVLO, or thermal shutdown. Device will remain
in FAULT mode until V
CC
power is cycled,
SKIP
is forced below ground (Figure 6), or
SHDN
is toggled.
1
AGND Switching
Run
(PFM/PWM)
1
X
High
Fault
APPROXIMATELY
-0.65V
1.5mA
V
FORCE
SKIP
AGND
(GND)
( ) ARE FOR THE MAX1714B ONLY.
MAX1714
Figure 6. Disabling Over/Undervoltage Protection
(No-Fault Test Mode)
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