MIC20XX_1108 Datasheet, PDF (21/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
Functional Description
V_IN and V_OUT
V_IN is both the power supply connection for the internal
circuitry driving the switch and the input (Source
connection) of the power MOSFET switch. V_OUT is the
Drain connection of the power MOSFET and supplies
power to the load. In a typical circuit, current flows from
V_IN to V_OUT toward the load. Since the switch is bi-
directional when enabled, if V_OUT is greater than V_IN,
current will flow from V_OUT to V_IN.
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_OUT exceed V_IN by
more than a diode drop (~0.6 V), while the switch is
disabled, current will flow from output to input via the
power MOSFET’s body diode.
If discharging C_LOAD is required by your application,
consider using MIC20X4 or MIC20X7; these MIC20XX
family members are equipped with a discharge FET to
insure complete discharge of C_LOAD.
Current Sensing and Limiting
MIC20XX 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 the 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
switch goes into thermal shutdown.
Kickstart™
2003 2004 2005X 2006 2007
2013 2014 2015 2016 2017
Only parts in bold have Kickstart™.
(Not available in 5-pin SOT-23 packages)
2008
2018
2009X
2019X
The MIC201X 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 Kickstart™ parts (MIC201X)
differs markedly from the non-Kickstart™ parts
(MIC200X) which immediately limit load current,
potentially starving the motor and causing the appliance
to stall or stutter.
MIC20xx Family
During this delay period, typically 128ms, a secondary
current limit is in effect. If the load demands a current in
excess the secondary limit, MIC201X acts immediately
to restrict output current to the secondary limit for the
duration of the Kickstart™ period. After this time the
MIC201X reverts to its normal current limit. An example
of Kickstart™ operation is shown in Figure 3.
Figure 3. Kickstart™ Operation
Figure 3 Label Key:
A. MIC201X is enabled into an excessive load (slew
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.
B. R_ON of the power FET increases due to internal
heating (effect exaggerated for emphasis).
C. Kickstart™ period.
D. Current limiting initiated. FAULT/ goes LOW.
E. V_OUT is non-zero (load is heavy, but not a dead short
where V_OUT = 0V. Limiting response will be the same
for dead shorts).
F. Thermal shutdown followed by thermal cycling.
G. Excessive load released, normal load remains.
MIC201X drops out of current limiting.
H. FAULT/ delay period followed by FAULT/ going
HIGH.
Undervoltage Lock-Out
Undervoltage lock-out insures no anomalous operation
occurs before the device’s minimum input voltage of
UVLO_THRESHOLD which is 2V minimum, 2.25V typical,
and 2.5V maximum 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.
August 2011
21
M9999-080211-D