MIC2095-2YMT(2010) Ver la hoja de datos (PDF) - Micrel
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componentes Descripción
Fabricante
MIC2095-2YMT Datasheet PDF : 25 Pages
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Micrel, Inc.
Functional Description and Application
Information
VIN and VOUT
VIN is both the power supply connection for the internal
circuitry driving the switch and the input (Source
connection) of the power MOSFET switch. VOUT is the
Drain connection of the power MOSFET and supplies
power to the load. In a typical circuit, current flows from
VIN to VOUT toward the load. Since the switch is bi-
directional when enabled, if VOUT is greater than VIN,
current will flow from VOUT to VIN.
When the switch is disabled, current will not flow to the
load, except for a small unavoidable leakage current of a
few micro amps. However, should VOUT exceed VIN by
more than a diode drop (~0.6V), while the switch is
disabled, current will flow from output to input via the
power MOSFET’s body diode. When the switch is
enabled, current can flow both ways, from VIN to VOUT, or
VOUT to VIN.
CIN
A minimum 1μF bypass capacitor positioned as close as
possible to the VIN and GND pins of the switch is both
good design practice and required for proper operation
of the switch. This will control supply transients and
ringing. Without a sufficient bypass capacitor, large
current surges or a short may cause sufficient ringing on
VIN (from supply lead inductance) to cause erratic
operation of the switch’s control circuitry. For best
performance a good quality, low-ESR ceramic capacitor
is recommended.
An additional 22μF (or greater) capacitor, positioned
close to the VIN and GND pins of the switch is necessary
if the distance between a larger bulk capacitor and the
switch is greater than 3 inches. This additional capacitor
limits input voltage transients at the switch caused by
fast changing input currents that occur during a fault
condition, such as current limit and thermal shutdown.
When bypassing with capacitors of 10μF and up, it is
good practice to place a smaller value capacitor in
parallel with the larger to handle the high frequency
components of any line transients. Values in the range of
0.1μF to 1μF are recommended. Again, good quality,
low-ESR capacitors, preferably ceramic, should be
chosen.
COUT
An output capacitor is recommended to reduce ringing
and voltage sag on the output during a transient
condition. A value between 1µf and 10µf is
recommended, however, larger values can be used.
MIC2095/97/98/99
Limitations on COUT
The part may enter current limit when turning on with a
large output capacitance. This is an acceptable
condition, however, if the part remains in current limit for
a time greater than tD_FAULT, the FAULT pin will assert
low. The maximum value of COUT may be approximated
by the following equation:
COUT _ MAX
=
ILIMIT _ MIN × tD _ FAULT _ MIN
VIN _ MAX
Eq. 1
Where: ILIMIT_MIN and tD_FAULT_MIN are the minimum
specified values listed in the Electrical Characteristic
table and VIN_MAX is the maximum input voltage to the
switch.
Current Sensing and Limiting
The current limiting switches protect the system power
supply and load from damage by continuously
monitoring current through the on-chip power MOSFET.
Load current is monitored by means of a current mirror
in parallel with the power MOSFET switch. Current
limiting is invoked when the load exceeds the over-
current threshold. When current limiting is activated the
output current is constrained to the limit value, and
remains at this level until either the load/fault is removed,
the load’s current requirement drops below the limiting
value, or the switch goes into thermal shutdown.
Kickstart™
The MIC2097 has a Kickstart feature that allows higher
momentary current surges before the onset of current
limiting. This permits dynamic loads, such as small disk
drives or portable printers to draw the inrush current
needed to overcome inertial loads without sacrificing
system safety. The Kickstart parts differ from the non-
Kickstart parts which more rapidly limit load current,
potentially starving a motor and causing the appliance to
stall or stutter.
During the Kickstart delay period, (typically 105ms), a
secondary current limit (nominally set at 1.5A), is in
effect. If the load demands a current in excess the
secondary limit, Kickstart parts act immediately to restrict
output current to the secondary limit for the duration of
the Kickstart period. After this time the Kickstart parts
revert to their normal current limit. An example of
Kickstart operation is in Figure 1.
Kickstart may be over-ridden by the thermal protection
circuit and if sufficient internal heating occurs, Kickstart
will be terminated and the output switch will be turned
off. After the parts cools, if the load is still present IOUT Æ
ILIMIT, not ILIMIT_2nd.
September 2010
16
M9999-092010-C
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