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| 型號(hào)： | 21045 |
| 英文描述： | Using the 3.3-V Am186ER or Am188ER Microcontroller in a 5-V System Application Note? 28.2KB (PDF) |
| 中文描述： | 使用5 V系統(tǒng)應(yīng)用指南3.3 V的Am186ER或Am188ER微控制器？ 28.2KB（PDF格式） |
| 文件頁(yè)數(shù)： | 2/3頁(yè) |
| 文件大?。?/td> | 28K |
| 代理商： | 21045 |
2
Using the 3.3-V Am186
ER or Am188
ER Microcontroller in a 5-V System
These devices are marketed in surface-mounted pack-
ages by a number of suppliers, and some devices are
even optimized directly for operation as a 5-V-to-3.3-V
converter. Cost of these devices is relatively low, com-
pared to the other solutions shown. Total cost, includ-
ing the cost of a bias current resistor, is less than $0.40
in single quantity surface mount packages.
Switching Regulators
The switching regulator solution can work well over a
large range of input voltages, including 5 V. The power
translation is usually very efficient; typical efficiency
ranges are from 85% to 95%. The negatives are that a
switcher may add noise problems (at the switching fre-
quency and the resulting harmonic frequencies), and
IC-based solutions typically require additional passive
(especially magnetic) components.
Switching regulator chips are available from compa-
nies such as Linear Technology, Maxim, Micro Linear,
and Motorola, among others. The driver IC typically in-
cludes the low-on-resistance boost-current FET device
on-chip, to minimize cost and complexity. However,
such solutions still require a passive magnetic compo-
nent (a transformer or inductor), and bulk current sup-
ply capacitor.
Costs for switching regulator solutions are the highest
among the three groups described. The total cost of a
switching regulator, including passive components,
can cost over $5, in OEM volume.
Modular switchers, sometimes known as DC-DC con-
verters, are complete switching regulator systems, in-
cluding all ICs, passives, and actives in a single potted
module. An example of such a device is the Power
Trends Modular 3.3-V converter, which includes
power-down controls, for low-power or shutdown appli-
cations, and is available in a small 12-pin SIP package.
These solutions are around $15 each, in volume.
Linear Regulators
Linear regulators provide tightly regulated output volt-
age levels in a simple single-chip solution. More specif-
ically, LDO (low-drop-out) regulators provide the
capability to generate 3.3 V from a 5-V supply. Manu-
facturers such as Linear Technology, Micrel, Semtech,
and National Semiconductor sell such devices. A dis-
advantage is that these devices are less efficient than
switchers, as they drop the difference voltage across
the pass transistor.
The cost of linear voltage regulators ranges from
around $1 to $2, even over the .5 to 1.5 A range. Addi-
tional costs include another bulk tantalum capacitor
(10 uF). (In either a 5-V or 3.3-V system, decoupling
capacitors are used).
Recommendation
For most designs, AMD recommends one of the many
low-dropout linear regulators. The Linear Technology
LT1129CST-3.3V is a good choice at 700-mA supply
current, SOT-223 SMT package, and a 400-mV drop-
out. Micrel provides the MIC2937A-3.3, also in SMT,
at a 750-mA current and 450-mV dropout. Any device
in this category would be appropriate for the 33-Mhz or
40-Mhz Am186ER microcontroller application. Other
linear regulators are available with various current ca-
pabilities and dropout voltages. The current require-
ments and maximum dropout voltage will be
determined by the particular design.
However, if efficiency is important, you may want to
use a switching regulator. And if the price of the com-
ponents is the ultimate consideration, then a zener
diode may be applicable. (The use of a zener diode is
extremely dependent on design-specific variables and
must be carefully evaluated and tested.)
Other Power Concerns
AMD recommends a split power plane, which isolates
the Am186ER device. The Am186ER should be in very
close proximity to the LDO regulator. Additional PCB
planes are not required.
A split plane is a portion of copper, cut out from the
PCB power plane (5-V layer) that powers only the 3.3-
V device, and should overlap the output pin of the reg-
ulator, thus providing low-impedance current supply.
Under no circumstances should the ground plane be
split; it provides a low impedance current sink and ref-
erence.
An output bulk capacitor of 1–10 uF, and distributed
capacitors of .01 uF for each power pin (as close to the
pin as possible) are recommended. If the regulator is
THT (through-hole technology), then the hole should
be directly connected to the power plane. If the regula-
tor is SMT (surface-mount technology), then the con-
nection to the power plane should be with large (>.050”
width) traces and through two (or more) vias, each with
.040” diameter holes and .060” diameter pads. Finally,
be sure that the regulator can easily handle the power
dissipation your solution implies. A 40-MHz design will
require approximately 220 mA, and the regulator will
drop 1.7 V, equaling a dissipation of approximately
375 mW of heat.
If compatibility must be maintained with the current 5-V
Am186EM/ES or Am188EM/ES microcontrollers, be
sure to allow a method of optionally reconnecting the
inner plane (microcontroller subplane) directly to the
external 5-V plane. The regulator would then be re-
moved or disabled, to allow direct 5-V operation. As-
sure a low-resistance connection from inner to outer
planes; it is recommended that a heavy gauge wire be
used to short the regulator input and output together.
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