MIC2004 Micrel, MIC2004 Datasheet - Page 11

• Manufacturer Part Number: MIC2004
• Description: Fixed Current Limit Power Distribution Switch
• Manufacturer: Micrel

August 2005
Micrel
Functional Description
Input and Output
V
circuitry driving the switch and the input (Source
connection) of the power MOSFET switch. V
Drain connection of the power MOSFET and supplies
power to the load. In a typical circuit, current flows from
V
directional when enabled, if V
current will flow from V
When the switch is disabled, current will not flow to the
load, except for a small unavoidable leakage current of
a few microamps. However, should V
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. While this effect can be
used to advantage when large bypass capacitors are
placed on MIC2004/2014’s’s output, it can not be relied
upon to fully or reliably discharge the load capacitance,
because discharging depends upon the characteristics
of the circuitry at VIN.
To ensure proper discharge of any output capacitance,
MIC2004/2014 is equipped with a discharge FET which
is ON any time the device is not Enabled.
Current Sensing and Limiting
MIC2004/2014 protects 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 an internally set 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 MIC2004/2014 goes into thermal shutdown.
Kickstart (MIC2014 only)
The MIC2014 is designed to allow momentary current
surges (Kickstart) before the onset of current limiting,
which permits dynamic loads, such as small disk drives
or portable printers to draw the energy needed to
overcome inertial loads without sacrificing system
safety. In this respect, the MIC2014 differs markedly
from MIC2004 and its peers, which immediately limit
load current, potentially starving the motor and causing
the appliance to stall or stutter.
During this delay period, typically 128 ms, a secondary
current limit is in effect. If the load demands a current in
excess the secondary limit, MIC2014 acts immediately
to restrict output current to the secondary limit for the
duration of the Kickstart period. After this time the
IN
IN
is both the power supply connection for the internal
to V
OUT
toward the load. Since the switch is bi-
OUT
to V
IN
OUT
.
is greater than V
OUT
exceed V
OUT
is the
IN
by
IN
,
11
MIC2014 reverts to its normal current limit. An example
of Kickstart operation is shown below.
A) MIC2014 is enabled into an excessive load (slew
B) R
C) Kickstart period.
D) Current limiting initiated. FAULT/ goes LOW (Note:
E) V
F) Thermal shutdown followed by thermal cycling.
G) Excessive load released, normal load remains.
H) FAULT/ delay period followed by FAULT/ going
Under Voltage Lock Out
Under voltage lock-out insures no anomalous operation
occurs before the device’s minimum input voltage of
2.5V had been achieved. Prior to reaching this voltage,
the output switch (power MOSFET) is OFF and no
circuit functions, such as FAULT/ or ENABLE, are
considered to be valid or operative.
Enable
ENABLE is a HIGH true control signal, which activates
Picture Key:
rate limiting not visible at this time scale) The initial
current surge is limited by either the overall circuit
resistance and power supply compliance, or the
secondary current limit, whichever is less.
heating (effect exaggerated for emphasis).
FAULT/ output is not available on MIC2014).
where V
for dead shorts).
MIC2014 drops out of current limiting.
HIGH. (FAULT/ output is not available on MIC2014).
O
O
U
OUT
ON
U
T
T
of the power FET increases due to internal
is non-zero (load is heavy, but not a dead short
Figure 3. Kickstart Operation
OUT
= 0. Limiting response will be the same
MIC2004/MIC2014
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