Micrel, Inc.
Enable Input
The MIC29311 version features an enable (EN) input
that allows on-off control of the device. Special design
allows “zero” current drain when the device is disabled—
only micro-amperes of leakage current flows. The EN
input has TTL/CMOS compatible thresholds for simple
interfacing with logic, or may be directly tied to V
IN
.
Enabling the regulator requires approximately 20µA of
current into the EN pin.
Error Flag
The MIC29311 features an error flag which looks at the
output voltage and signals an error condition when this
voltage drops 5% below its expected value. The error
flag is an open-collector output that pulls low under fault
conditions. It may sink 10mA. Low output voltage
signifies a number of possible problems, including an
overcurrent fault (the device is incurrent limit) and low
input voltage. The flag output is inoperative during
overtemperature shutdown conditions.
USB Applications
The main application of the MIC29311 is to control
power distribution in a self-powered Universal Serial Bus
hub. For self-powered hubs, the MIC29311 provides
5.1V ±3% to downstream ports from an unregulated
supply voltage (see “Typical Application”). USB requires
that the downstream voltage supplied to peripherals from
a self-powered hub is between 4.75V and 5.25V. The
MIC29311 provides regulation with this requirement. The
enable pin input controls ganged power for up to seven
downstream ports, each drawing up to 500mA. The
device has an output current limiting circuit that linearly
decreases the output voltage as the output current
exceeds 3A. When the part is out of regulation by 5%,
the error flag goes low and signals a fault condition to
the microcontroller, allowing the system to be disabled.
This provides the overcurrent protection that is required
by USB.
MIC29311
In Figure 2, the MIC29311 provides power to the
MIC2527 quad power switches. The MIC2527 provides
power switching to four independently controlled
downstream ports. Two MIC2527’s can be used to
provide a 6-port, self-powered hub in conjunction with
the MIC29311. The 5.1V output of the MIC29311 is
optimized to ensure that under maximum load condition,
the output voltage of each channel of the MIC2527
remains above 4.75V. This is the minimum voltage
requirement for self-powered USB hubs. The output
voltage is a function of the minimum output voltage of
the power supply, the PCB trace resistance and the on-
resistance of the switch. Table 1 shows the maximum
allowable on-resistance for a 5.1V power supply in a
self-powered hub, assuming 30mV of voltage drop due
to PCB trace resistance. The 5.1V of the MIC29311 and
the 300mΩ on-resistance of each MIC2527 switch
provide an economical solution to power management of
self-powered hubs.
Nominal
Voltage
Supply
Tolerance
1%
2%
5.10V
3%
4%
5%
Minimum
Voltage
5.05V
5V
4.95V
4.9V
4.85V
Maximum
Voltage
5.15V
5.2V
5.25V
5.3V
5.38V
Maximum
R
ON
540mΩ
440 mΩ
340 mΩ
–
–
Table 1. Max. Allowable On-Resistance
For further information concerning USB power
management, refer to Application Note 17 and
Application Hint 30.
September 2007
9
M9999-090507
MICREL [ MICREL SEMICONDUCTOR ]
