MIC2017 Micrel Semiconductor, MIC2017 Datasheet - Page 11

• Manufacturer Part Number: MIC2017
• Description: (MIC2007 / MIC2017) Adjustable Current Limit Power Distribution Switch
• Manufacturer: Micrel Semiconductor

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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 MIC2007/2017’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,
MIC2007/2017 is equipped with a discharge FET which
is ON any time the device is not Enabled.
Current Sensing and Limiting
The MIC2007/2017 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 externally 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 MIC2007/2017 goes into
thermal shutdown.
Kickstart (MIC2017 only)
The MIC2017 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 MIC2017 differs markedly
from MIC2007 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 of the secondary limit, the MIC2017 acts
immediately to restrict output current to the secondary
October 2005
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
limit for the duration of the Kickstart period. After this
time, the MIC2017 reverts to its normal current limit. An
example of Kickstart operation is shown below.
Picture Key:
A) MIC2017 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 ENABLE, are considered to be
valid or operative.
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).
MIC2007/2017 does not provide a FAULT/ output.)
where V
for dead shorts).
MIC2017 drops out of current limiting.
HIGH. (Note: MIC2007/2017 does not provide a
FAULT/ output.)
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
MIC2007/MIC2017
M9999-102805