MIC2529 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Single-Channel USB Power Controller Advance Information

MIC2529
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located at
the device, is strongly recommended to control supply tran-
sients. See Figure 1. Without a bypass capacitor, an output
short may cause sufficient ringing on the input (from supply
lead inductance) to damage internal control circuitry.
Input or output transients must not exceed the absolute
maximum supply voltage (V_IN(max) = 6V) even for a short
duration.
4.0V to 5.5V
MIC2529
1 _FLG
EN ⁸
2 _OUT
OUT ⁷
3 _IN
3VEN ⁶
0.1µF
to
1µF
4 _{3VOUT GND} 5
1µF
Figure 1. Supply Bypassing
Regulator Output Capacitor
A 1µF output capacitor is required between 3VOUT and GND
to prevent oscillation. Larger values improve the regulator’s
transient response. The output capacitor value may be in-
creased without limit.
The output capacitor should have an ESR (effective series
resistance) of about 5Ω or less and a resonant frequency
above 1MHz. Most tantalum or aluminum electrolytic capaci-
tors are adequate. Film types will work but are more expen-
sive. Since many aluminum electrolytics have electrolytes
that freeze at about –30°C, solid tantalums are recom-
mended for operation below –25°C.
The regulator will remain stable and in regulation with no load
other than the internal voltage divider, unlike many other
voltage regulators.
Switch Enable Input
EN must be driven logic high or logic low for a clearly defined
input. Floating the input may cause unpredictable operation.
EN should not be allowed to go negative with respect to GND.
Regulator Enable Input
Forcing 3VEN (regulator enable/shutdown) high enables the
regulator. Like EN, 3VEN is compatible with CMOS logic
gates.
If the enable/shutdown feature is not required, connect 3VEN
to IN.
Transient Overcurrent Fault-Flag Filter
When the MIC2529 is enabled, large values of capacitance
at the output of the device may cause inrush currents to flow
that exceed the short circuit current-limit threshold of the
device and cause the flag to activate. The duration of this time
depends on the size of the output capacitance. Refer to
Micrel
“Functional Characteristics: Switch Turn-On and Turnoff
Characteristics” for details. During the charging time, the
device enters into constant-current mode and the flag is
activated. As the capacitance is charged, the current de-
creases below the short circuit current-limit threshold and the
flag will be deasserted.
V+
USB Controller
OVERCURRENT
ON/OFF
10k
10k
0.1µF
MIC2529
1 _FLG
EN ⁸
2 _OUT
OUT ⁷
3 _IN
3VEN ⁶
4 _{3VOUT GND} 5
Figure 2. Transient Filter
In USB applications it is required that output bulk capaci-
tance, shown in “Typical Application,” is utilized to meet
transient regulation requirements during hot-plug events.
When the MIC2529 is enabled into this capacitive load, the
flag may go active for about 200µs. Additionally, during hot-
plug events, inrush currents will also cause the flag to go
active for approximately 30µs. Since these conditions are not
valid overcurrent faults, the USB controller must ignore the
flag during these events. To prevent this erroneous overcur-
rent reporting, a 1ms RC filter must be used (see Figure 2).
Bus-Powered Hub Applications
The MIC2529 was designed for self-powered or bus-pow-
ered hubs. The integrated regulator reduces cost and space
in applications using a 3.3V USB controller. For self-powered
hubs, the MIC2529 will support 1 downstream port. For bus-
powered hubs, the MIC2529 will support up to four ganged
downstream ports.
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 does
have some 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 require-
ment, the MIC2529 has been designed to slowly ramp its
output.
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 MIC2529
provides a flag output for this application.
June 1999
9
MIC2529