參數(shù)資料
型號(hào)： MAX1816ETM
廠商： MAXIM INTEGRATED PRODUCTS INC
元件分類(lèi)： 穩(wěn)壓器
英文描述： Replaced by TMS320VC5506 : Digital Signal Processors 144-LQFP
中文描述： DUAL SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, CQCC48
封裝： 7 X 7 MM, 0.80 MM HEIGHT, THIN, QFN-48
文件頁(yè)數(shù)： 37/49頁(yè)
文件大小： 983K
代理商： MAX1816ETM
M
Dual Step-Down Controllers Plus Linear-
Regulator Controller for Notebook Computers
______________________________________________________________________________________
37
To allow startup, UVP is ignored during the undervoltage
blanking time (the first 256 cycles of the slew rate after
startup for BUCK1, the first 4096 cycles for BUCK2 and
the first 512 cycles for the linear regulator). UVP can be
disabled using the NO FAULT test mode (see the
NO
FAULT Test Mode
section).
UVLO
The MAX1816/MAX1994 provide input undervoltage lock-
out (UVLO) protection. If the V
CC
voltage drops low
enough to trip the UVLO comparator, it is assumed that
there is not enough supply voltage to make valid deci-
sions. In order to protect the output from overvoltage
faults, DL1 and DL2 are forced high if OVP is enabled,
DH_ is forced low, and the linear regulator is turned off. If
OVP is disabled, DL1 is forced high, DL2 is forced low,
DH_ is forced low, and the linear regulator is turned off.
For BUCK1 (and also for BUCK2 if OVP is enabled), this
condition rapidly forces the outputs to zero since the
slew-rate controller is not active. The fault results in large
negative inductor currents and possibly small negative
output voltages. If V
CC
is likely to drop in this fashion, the
outputs can be clamped with Schottky diodes to PGND to
reduce the negative excursions.
Thermal Fault Protection
The MAX1816/MAX1994 feature a thermal fault-protec-
tion circuit. When the junction temperature rises above
+160
°
C, a thermal sensor sets the fault latch, which
pulls DL_ high, DH_ low, and turns off the linear regula-
tor. The device remains in fault mode until the junction
temperature cools by 15
°
C, and either V
CC
power is
cycled, or SKP_/
SDN
or LIN/
SDN
is toggled.
NO FAULT Test Mode
The over/undervoltage protection features can compli-
cate the process of debugging prototype breadboards
since there are (at most) a few milliseconds in which to
determine what went wrong. Therefore, a test mode is
provided to disable the OVP, UVP, and thermal shut-
down features, and clear the fault latch if it has been
set. Test mode applies to BUCK1, BUCK2, and the lin-
ear regulator. In the test mode, BUCK1 operates as if
SKP1/
SDN
was high (skip mode). Set the voltage on
SKP1/
SDN
between 10.8V to 13.2V to enable the NO
FAULT test mode.
BUCK1/BUCK2
Design Procedure
Firmly establish the input voltage range and maximum
load current for BUCK1 and BUCK2 before choosing a
switching frequency and inductor operating point (rip-
ple-current ratio). The primary design trade-off lies in
choosing a good switching frequency and inductor
operating point, and the following four factors dictate
the rest of the design:
1)
Input Voltage Range.
The maximum value
(V
IN(MAX)
) must accommodate the worst-case high
AC adapter voltage. The minimum value (V
IN(MIN)
)
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery selec-
tor switches. If there is a choice, lower input volt-
ages result in better efficiency.
2)
Maximum Load Current.
There are two values to
consider. The peak load current (I
LOAD(MAX)
) deter-
mines 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 continu-
ous load current (I
LOAD
) determines the thermal
stresses and thus drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components. Modern notebook CPUs gen-
erally exhibit I
LOAD
= I
LOAD(MAX)
80%.
3)
Switching Frequency.
This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses that
are proportional to frequency and V
IN
. The optimum
frequency is also a moving target, due to rapid
improvements in MOSFET technology that are mak-
ing higher frequencies more practical.
4)
Inductor Operating Point.
This choice provides
tradeoffs between size and 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
touches zero with every cycle at maximum load).
Inductor values lower than this grant no further size-
reduction benefit. The MAX1816/MAX1994s
’
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.
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