Micrel, Inc.
MIC2026/2076
Universal Serial Bus (USB) Power Distribution
Application Information
The MIC2026/76 are ideally suited for USB (Universal
Serial Bus) power distribution applications. The USB
specification defines power distribution for USB host
systems such as PCs and USB hubs. Hubs can either
be self-powered or bus-powered (that is, powered from
the bus). Figure 5 shows a typical USB Host application
that may be suited for mobile PC applications employing
USB. The requirement for USB host systems is that the
port must supply a minimum of 500mA at an output
voltage of 5V ±5%. In addition, the output power
delivered must be limited to below 25VA. Upon an
overcurrent condition, the host must also be notified. To
support hot-plug events, the hub must have a minimum
of 120µF of bulk capacitance, preferably low ESR
electrolytic or tantulum. Please refer to Application Note
17 for more details on designing compliant USB hub and
host systems.
Supply Filtering
A 0.1µF to 1µF bypass capacitor positioned close to VIN
and GND of the device is strongly recommended to
control supply transients. Without a bypass capacitor, an
output short may cause sufficient ringing on the input
(from supply lead inductance) to damage internal control
circuitry.
Printed Circuit Board Hot-Plug
The MIC2026/76 are ideal inrush current-limiters for hot
plug applications. Due to their integrated charge pumps,
the MIC2026/76 present a high impedance when off and
slowly become a low impedance as their integrated
charge pumps turn on. This “soft-start” feature effectively
isolates power supplies from highly capacitive loads by
reducing inrush current. Figure 3 shows how the
MIC2076 may be used in a card hot-plug application.
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
addition, to reduce voltage droop on the upstream VBUS,
soft-start is necessary. Although the hub can consume
up to 500mA from the upstream bus, the hub must
consume only 100mA max at start-up, until it
enumerates with the host prior to requesting more
power. The same requirements apply for bus-powered
peripherals that have no downstream ports. Figure 6
shows a bus-powered hub.
In cases of extremely large capacitive loads (>400µF),
the length of the transient due to inrush current may
exceed the delay provided by the integrated filter. Since
this inrush current exceeds the current-limit delay
specification, FLG will be asserted during this time. To
prevent the logic controller from responding to FLG
being asserted, an external RC filter, as shown in Figure
4, can be used to filter out transient FLG assertion. The
value of the RC time constant should be selected to
match the length of the transient, less tD(min) of the
MIC2026/76.
USB
Controller
SPN010012
USB
1
8
7
6
5
ENA OUTA
VBUS
Function
2
FLGA
IN
4.7
CBULK
µF
3
to "Hot"
FLGB GND
ENB OUTB
Receptacle
USB
Function
4
CBULK
GND
USB Peripheral
Cable
Figure 3. Hot-Plug Application
M9999-060410-B
June 2010
13
MICROCHIP [ MICROCHIP ]
