MIC2536
Universal Serial Bus (USB) Power Distribution
Applications
The MIC2536 is ideally suited for USB (Universal Serial Bus)
power distribution applications. For Bus-Powered hubs, USB
requires that each downstream port be switched on or off
under control by the host. Up to four downstream ports each
capable of supplying 100mA at 4.4V minimum are allowed. In
additon, to reduce voltage droop on the upstream bus the hub
must consume only 100mA max at start-up until it enumer-
ates with the host prior to requesting more power. The same
requirements apply for bus-powered peripherals that have no
downstream ports. Figure 4 shows a two-port bus-powered
hub.
Bus-Powered Hub Port Switching
The USB Specification requires that bus-powered hubs imple-
ment port switching on either a ganged or individual basis.
The specific implementation must be reported via the Hub
Descriptor Status Register. Individual port switching has
advantages in that a fault on one port will not prevent the other
ports from operating correctly. In addition, a soft-start circuit
must be included in order to reduce inrush currents when the
switch is enabled. To meet this requirement, the MIC2536
has been designed to slowly ramp its output.
Suspend Current
Universal Serial Bus Specification
places a maximum sus-
pend current requirement of 500µA on devices. For hubs,
Universal Serial Bus Specification Revision 1.1
clarifies this
issue. Revision 1.1, section 7.2.3, stipulates that the maxi-
mum suspend current for a configured hub is 2.5mA. This
number is derived by allocating 500µA for up to four down-
stream ports plus 500µA for the hub’s internal functions. A
nonconfigured hub is considered a low-power device and
Micrel
may not consume more than 500µA. In a nonconfigured state
all downstream devices will be switched off. In most cases, a
nonconfigured hub is not a practical state for the system.
Therefore, the 2.5mA specification is the applicable target
specification for the suspend state. In a bus-powered hub
with less than 4 ports, the hub may use the additional current
for internal functions.
The 500µA worst case suspend current must be further
divided among the data port termination resistors and internal
functions. The termination resistors will consume
3.6V
÷
(16.5KΩ – 5%) = 230µA. This leaves only 270µA for
internal functions. Assuming 100µA as the maximum USB
controller suspend current, 170µA remains for the rest of the
system. The MIC2536 will consume 100µA maximum, leav-
ing a margin of 70µA.
USB Voltage Regulation
USB specifications require a minimum downstream voltage
supply of 4.40V from a bus-powered hub port (See
Applica-
tion Note 17
for details). The USB specification allows for a
100mV drop across the hub, leaving 250mV for PCB, up-
stream cable, and connector resistance. Therefore, the on-
resistance of the switch for each port, not including PCB
resistance, must be <100mV
÷
100mA = 1Ω. The MIC2536
has a maximum on-resistance of 700mΩ, which easily satis-
fies this requirement.
Overcurrent Indication
The USB Specification does not require bus-powered hubs to
report overcurrent conditions to the host, since the hub is
already current-limited at the upstream port. However, if it is
desired to report overcurrent, the Hub Descriptor Status
Register must be programmed to indicate this. The MIC2536
provides a flag output for this application.
10k
4.50V to 5.25V
Upstream V
BUS
100mA max.
V
BUS
D+
D–
GND
4.7µF
1.5k
MIC5207-3.3
IN
OUT
GND
1µF
10k
3.3V USB Controller
VIN
ON/OFF
OVERCURRENT
OVERCURRENT
ON/OFF
MIC2536-2
ENA
FLGA
FLGB
ENB
OUTA
IN
GND
OUTB
63µF
0.1µF
Ferrite
Beads
V
BUS
D+
.01µF
D–
GND
USB
Port 1
V
BUS
D+
.01µF
63µF
D–
GND
Data
(Two Pair)
to USB
Controller
USB
Port 2
Data
Figure 4. USB Two-Port Bus-Powered Hub
March 2000
11
MIC2536
MICREL [ MICREL SEMICONDUCTOR ]
