APPHINT30 [ETC]
MIC2527 Voltage Drop. Packaging. and PCB Layout ; MIC2527的电压降。包装。和PCB布局\n
| 型号： | APPHINT30 |
| 厂家： | 
ETC |
| 描述： | MIC2527 Voltage Drop. Packaging. and PCB Layout
|
| 文件： | 总2页 (文件大小：47K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Application Hint 30
MIC2527 Voltage Drop, Packaging and PCB Layout
by Kris Jones and Kevin Lynn
The MIC2527 was designed to provide cost-effective indi-
vidual port protection and switching for USB self-powered
hub designs. Analysis of voltage drops under several design
scenarios shows that the most economical approach to
meetingUSBvoltagerequirementsistousea300mΩ switch
and a 3% power supply “biased up” to 5.1V. Most USB
controllers can also operate with this supply since they are
expected to operate from 4.0V to 5.25V.
Self-Powered Hub Design
The output voltage requirement for USB self-powered hubs
is 4.75V minimum to 5.25V maximum under no-load and
maximum-load (500mA) conditions. The output voltage is a
functionofpowersupplyvoltageandtolerance, PCBconnec-
tor and trace resistances, and switch resistance:
4.75V (min) =
V
(Power Supply) – V
(PCB) – V
DROP DROP
(Switch)
MIN
Nominal Supply
Minimum Maximum Maximum
Todeterminethesetofpowersupplyvoltagesandtolerances
which fall within the USB requirement, minimum and maxi-
mum output voltages were calculated for nominal supplies in
the range of 4.85V to 5.15V and with 1% to 5% tolerances.
Voltage Tolerance Voltage
Voltage
R
ON
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
1%
2%
3%
4%
5%
4.8V
4.75V
4.7V
4.9V
40mΩ
0mΩ
—
4.95V
5V
4.85V
4.90V
4.95V
5.00V
5.05V
5.10V
5.15V
See Table 1. Power supplies which have V
< 4.75V or
MIN
4.66V
4.61V
4.85V
4.8V
5.04V
5.09V
4.95V
5V
—
V
> 5.25V cannot be used for USB applications. Note
MAX
—
that, even for a supply centered at 5V, the supply tolerance
must be better than 5% to allow for any losses due to PCB
connector and trace resistance.
140mΩ
40mΩ
0mΩ
—
4.75V
4.7V
5.05V
5.1V
30mV is generally sufficient to account for voltage drops due
to PCB connector and trace resistance. For recommenda-
tions to minimize PCB connector and trace losses through
proper board layout and design, please refer to Application
Note 17 “Universal Serial Bus Power Management.”
4.66V
4.9V
5.15V
5V
—
240mΩ
140mΩ
40mΩ
0mΩ
—
4.85V
4.8V
5.05V
5.1V
Using minimum power supply output voltages and a 30mV
drop for the PCB, we can calculate the maximum on-resis-
tance required for the switch as follows:
4.75V
4.7V
5.15V
5.2V
4.95V
4.9V
5.05V
5.1V
340mΩ
240mΩ
140mΩ
40mΩ
0mΩ
440mΩ
340mΩ
240mΩ
140mΩ
—
V
− 4.75V − 0.03V
MIN
R
switch (max) =
Ω
ON
0.5A
4.85V
4.8V
5.15V
5.2V
Calculated values for maximum switch resistance are shown
in Table 1 for all usable power supply ranges. Power supply
4.75V
5V
5.25V
5.1V
ranges requiring R to be 0Ω are also not usable for USB
ON
since some voltage drop must be reserved for the switch.
These calculations show that as the nominal power supply is
increased, higher values of switch resistance, and therefore
lower cost switches, can be used.
4.95V
4.9V
5.15V
5.2V
4.85V
4.8V
5.25V
5.3V
A 3% power supply tolerance generally provides a good
compromise between accuracy and cost. For the usable
power supply ranges in Table 1, the most economical switch
(340mΩ maximum) can be used with a 5.1V, ±3% supply.
The MIC2527, with 300mΩ maximum on-resistance, was
designed to meet this requirement.
5.05V
5V
5.15V
5.2V
540mΩ
440mΩ
340mΩ
—
4.95V
4.9V
5.25V
5.3V
4.85V
5.1V
5.36V
5.2V
—
The 5.1V 3% supply can be generated using a Micrel
MIC29311-5.1BT voltage regulator. If a 5V, ±3% supply must
be used, the MIC2524 with 140mΩ on-resistance is ideally
suited.
640mΩ
540mΩ
—
5.05V
5V
5.25V
5.3V
4.94V
4.89V
5.36V
5.41V
—
—
Table 1. Maximum Allowed On-Resistance
with 30mV PCB Voltage Drop
Shading represents USB-compliant conditions.
September 1999
1
Application Hint 30
Application Hint 30
Micrel
MIC29311-5.1
LDO Regulator
5.7V
Ferrite
Bead
5.1V ±3%
4.75V min.
at 500mA
IN
OUT
ERR
GND
EN
VBUS
D+
10k
47k
Downstream
33µF*
USB
Port 1
3.3V USB Controller
V+
OVERCURRENT
MIC2527
0.01µF
D–
MIC5207-3.3
LDO Regulator
IN
ON/OFF
ENA
500mA max.
GND
FLGA
ENB
IN
IN
OUT
0.1
µF
OUTA
4.7
µF
1µF
VBUS
D+
FLGB
ENC
OUTB
OUTC
GND
Downstream
USB
33µF*
33µF*
33µF*
0.01µF
0.01µF
0.01µF
D+
D–
D–
Port 2
FLGC OUTD
500mA max.
GND
GND
END
GND
GND
FLGD
VBUS
D+
Bold lines indicate
0.1" wide, 1-oz. copper
high-current traces.
Downstream
USB
D–
Port 3
500mA max.
GND
* 33µF, 16V tantalum or 100µF, 10V electrolytic per port
VBUS
D+
Downstream
USB
D–
Port 4
500mA max.
GND
Figure 1. MIC2527 Application
Heat Sink Requirements
To determine regulator heat sink requirements, calculate the
regulator power dissipation at the applicable input voltage:
0.050
DIMENSIONS:
INCHES
P = I
(1.02 × V – V
)
OUT
D
OUT
IN
0.200
0.250
where:
V
< 7.0V
IN
At V = 7V:
0.375
IN
P
= 2.1A (1.02 × 7V – 5.1V)
D(max)
0.020 MIN.
BETWEEN
PADS
0.450
P
= 4.3W
D(max)
Using the same formula for V = 5.4V, the minimum input
IN
voltage, P
is 0.86W. For further information, see the
D(max)
MIC29311 data sheet.
0.030 MAX.
PAD WIDTH
IftheaverageV isgreaterthan7V,a3Astepdownswitching
IN
regulator, such as the MIC4576, may replace the linear
regulator, with reduced heat sink requirements.
Packaging and Board Layout
0.025 MIN.
TRACE WIDTH
The MIC2527 is offered in the 16-pin plastic DIP package for
through-hole mounting and in the 16-pin 0.300-inch wide
SOIC package for surface mounting. Micrel plans to add a
third package option with the 16-pin 0.150-inch narrow SOIC
package.
0.050 (BASIC)
PAD CENTERS
Narrow SO-16 (M)
Wide SO-16 (WM)
For customers who would like to migrate from the 0.300-inch
to the 0.150-inch SOIC package, it is possible to layout the
PCB to take either package by using longer traces to the
package leads.
Figure 2. Dual-Package PCB Layout
Figure 2 shows the nominal trace dimensions needed for a
dual 0.150-inch/0.300-inch SOIC layout.
Application Hint 30
2
September 1999
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