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| 型號： | APPNOTE17 |
| 英文描述： | Universal Serial Bus Power Management |
| 中文描述： | 通用串行總線電源管理 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大?。?/td> | 125K |
| 代理商： | APPNOTE17 |
Application Note 17
Circuit Examples
Figure 5 shows a minimal self-powered hub, using Micrel’s
MIC29301-5.0 3A LDO linear regulator to provide 5.0V
±
2%
(4.9V to 5.1V) from an unregulated>5.35V, 3.5A transformer-
rectifier-capacitor input. The MIC29301-5.0BT has an enable
pin to control ganged power, up to 3.5A, to seven down-
stream ports each drawing up to 500mA. The MIC29301
limits current to less than 4A with less than 5A short-circuit
current. An error flag goes low if the device is in current limit,
thermal shutdown, or has an excessive output voltage drop.
If heavy loads cause the thermal shutdown to activate, a slow
thermal oscillation may occur as the regulator heats up under
load and cools down when shut-off. The hub controller may
be programmed to shut down the LDO regulator after the
error flag has been low for a preset time and then to periodi-
cally reenergizing the LDO regulator enable to determine if
the excessive load has been removed.
Output Filter
Output ports must have a 120
μ
F bulk capacitance per hub.
Tantalum capacitors across the power and ground lines, near
each downstream connector, reduce EMI and decouple
voltage droop caused when downstream cables are hot-
attached. Ferrite beads in series with V
bus
, the ground line
and the 0.01
μ
F bypass capacitors at the power connector
pins are recommended for EMI and ESD protection.
Flag Transient Filtering
The transient in-rush current to downstream capacitance
may cause a short-duration (10
μ
s to 20
μ
s) error flag, which
may cause erroneous overcurrent reporting. A simple 1ms
RC low-pass filter (10k
and 0.1
μ
F) in the flag line (see
Figures 5, 6, and 7) eliminates short-duration transients.
Electrostatic Discharge Protection
Flyback transients and electrostatic discharge (ESD) may
exceed the maximum operating voltage ratings of ICs. Micrel’s
MIC2526 has been tested and certified to exceed the require-
ments of IEC 1000-4-2 (EN 50082-1) for 15kV air-discharge
and 8kV contact discharge with the output filters shown in the
circuit examples (120
μ
F, 0.01
μ
F, and ferrite beads).
Micrel
Application Note 17
8
October 1998
Although Micrel’s USB switches are compliant to IEC stan-
dards for ESD protection, USB controllers may need addi-
tional ESD protection to assure a robust design. For USB
controller data lines, ESD protection should also be provided
protection that can be added inexpensively with external
transient protection devices such as the SurgeX protector
from Cooper/Bussman, telephone (314) 394-2877.
Self-Powered Hub with Individual Power Switches
When a local 5V
+5
–3
% power supply, such as a standard
personal computer, provides operating voltage from 4.85 to
5.25V, protection devices with less voltage drop than a
polyfuse or LDO regulator are needed. Micrel’s integrated
high-side power switches have low on-resistance, built-in
current limiting, thermal shutdown, and logic-level enable.
The worst-case voltage drop recommended across the self-
powered hub board, MIC2526, and ferrite beads is 100mV,
guaranteeing 4.75V at the downstream output port.
Figure 6 illustrates a self-powered hub with up to four down-
stream 500mA switched ports, using two MIC2526 dual 0.5A
power switches.
Figure 7 shows a four-port self-powered hub, using a single
MIC2505 2A power switch to provide ganged power from a
5V
+5
–3
% regulated local supply.
Ganged-Output Bus-Powered Hub
Figure 8 shows a single MIC2525 140m
switch providing
ganged power switching to four downstream ports. The
single power switch controls all downstream ports, with soft
start limiting the inrush current to the four downstream 33
μ
F
capacitors when the switch is enabled. A low-cost MIC5203
(SOT-143) or MIC5207 (TO-92) 3.3V low-dropout (LDO)
regulator provides power from the bus to the USB controller.
Summary
Micrel’s logic-controlled high-side power switch integrated
circuits are ideal for USB power switching applications. Each
offer superior protection and performance relative to polyfuses.
In addition, proper PCB layout techniques are highly recom-
mended to ensure that both USB voltage regulation and
transient regulation specifications are satisfied.
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