鈥?/div>
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)
鈥?V
DROP (PCB)
鈥?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 鈥淯niversal Serial Bus Power Management.鈥?/div>
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:
V
MIN
鈭?/div>
4.75V
鈭?/div>
0.03V
鈩?/div>
0.5A
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鈩?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.
R
ON
switch (max)
=
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鈩?maximum) can be used with a 5.1V,
鹵3%
supply.
The MIC2527, with 300m鈩?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,
鹵3%
supply must
be used, the MIC2524 with 140m鈩?on-resistance is ideally
suited.
Table 1. Maximum Allowed On-Resistance
with 30mV PCB Voltage Drop
Shading represents USB-compliant conditions.
September 1999
1
Application Hint 30
1
2
