MIC2537_11 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – Quad Power Distribution Switch Quad Power Distribution Switch

MIC2537
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. Without a bypass capacitor, an output short may
cause sufficient ringing on the input (from supply lead induc-
tance) 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.
V_IN
3V to 5.5V
0.1µF to 1µF
MIC2537
FLGA FLGB
ENA ENB
OUTA OUTB
GND
IN
NC
NC
OUTC OUTD
ENC END
FLGC FLGD
Figure 1. Supply Bypassing
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.
Soft Start
The MIC2537 presents a high impedance when off and slowly
becomes a low impedance as it turns on. This reduces inrush
current and related voltage drop that results from charging a
capacitive load, satisfying the USB voltage droop require-
ments.
Transient Overcurrent Fault-Flag Filter
When the MIC2537 is enabled, large values of capacitance
at the output of the device will 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. See “Func-
tional Characteristics: Switch Turn-On and Turnoff
Charcteristics” 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.
Micrel
In USB applications it is required that output bulk capacitance
shown in “Typical Application” is utilized to meet transient
regulation requirements during hot-plug events. When the
MIC2537 is enabled, the flag will go active for about 5ms
depending on output capacitance. Additionally, during hot-
plug events, inrush currents may cause the flag to go active
for approximately 30µs. If these “false” overcurrent indica-
tions are a system problem, they can be masked by an RC
filter on the flag output (see Figure 2). Alternatively, a 15ms
debounce routine may be programmed into the USB logic
controller to eliminate the need for an RC filter.
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 MIC2537
has been designed to slowly ramp its output.
Suspend Current
For hubs, Universal Serial Bus Specification Revision 1.1
section 7.2.3, stipulates that the maximum suspend current
for a configured hub is 2.5mA. This number is derived by
allocating 500µA for up to four downstream ports plus 500µA
for the hub’s internal functions. A nonconfigured hub is
considered a low-power device and cannot 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 speci-
fication 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 MIC2537 will consume 100µA maximum, leav-
ing a margin of 70µA.
USB Controller
OVERCURRENT
10k
50k
0.3µF
FLGA
FLGB
FLGC
FLGD
Figure 2. Transient Filter
December 1999
9
MIC2537