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| 型號： | APPHINT30 |
| 英文描述： | MIC2527 Voltage Drop. Packaging. and PCB Layout |
| 中文描述： | MIC2527壓降。包裝。和PCB布局 |
| 文件頁數(shù)： | 1/2頁 |
| 文件大?。?/td> | 43K |
| 代理商： | APPHINT30 |
September 1999
1
Application Hint 30
Application Hint 30
Micrel
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
meeting USB voltage requirements is to use a 300 m
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.
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Table 1. Maximum Allowed On-Resistance
with 30mV PCB Voltage Drop
Shading represents USB-compliant conditions.
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
function of power supply voltage and tolerance, PCB connec-
tor and trace resistances, and switch resistance:
4.75V (min) =
V
MIN
(Power Supply)
– V
DROP
(PCB)
– V
DROP
(Switch)
To determine the set of power supply voltages and tolerances
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.
See Table 1. Power supplies which have V
MIN
< 4.75V or
V
MAX
> 5.25V cannot be used for USB applications. Note
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.
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.”
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:
R
switch (max)
V
4.75V
0.5A
0.03V
ON
MIN
=
Calculated values for maximum switch resistance are shown
in Table 1 for all usable power supply ranges. Power supply
ranges requiring R
ON
to be 0
are also not usable for USB
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.
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
(340m
maximum) can be used with a 5.1V,
±
3% supply.
The MIC2527, with 300m
maximum on-resistance, was
designed to meet this requirement.
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 140m
on-resistance is ideally
suited.
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