MIC2026A_09 Datasheet, PDF (13/16 Pages) Micrel Semiconductor – Dual-Channel Power Distribution Switch

Micrel, Inc.
MIC2026A/2076A
Figure 2. MIC2026A Fault Timing
Application Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor positioned close to V_IN
and GND pins 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 MIC2026A/2076A are ideal inrush current-limiters
for hot plug applications. Due to their integrated charge
pumps, the MIC2026A/2076A 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 MIC2026A may be used in a card hot-plug
application.
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 t_D(min) of the
MIC2026A/2076A.
Universal Serial Bus (USB) Power Distribution
The MIC2026A/2076A 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 tantalum. Please refer to Application
Note 17 for more details on designing compliant USB
hub and host systems.
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 V_BUS,
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.
July 2009
13
M9999-072309-B
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