MIC2090_11 Datasheet, PDF (15/23 Pages) Micrel Semiconductor – 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.
When the switch is disabled, current will not flow to the
load, except for a small unavoidable leakage current of a
few microamps (forward leakage current).
C_IN
A minimum 1μF bypass capacitor positioned close to the
V_IN 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 V_IN (from supply
lead inductance) to cause erratic operation of the
switch’s control circuitry. For best performance, place a
ceramic capacitor next to the IC.
An additional 10µF (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 three 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 or more, 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.
C_OUT
An output capacitor is required to reduce ringing and
voltage sag on the output during a transient condition. A
value between 1μF and 10μF is recommended.
A 10μF or larger capacitor should be used if the distance
between the MIC2090/MIC2091 and the load is greater
than three inches. The internal switch in the
MIC2090/MIC2091 turns off in (typically) 20ns. This
extremely fast turn-off can cause an inductive spike in
the output voltage when the internal switch turns off
during an overcurrent condition. The larger value
capacitor prevents the output from glitching too low.
MIC2090/MIC2091
Limitations on C_OUT
The part may enter current limit when turning on with a
large output capacitance, which is an acceptable
condition. However, if the part remains in current limit for
a time greater than t_{D_FAULT}, the FAULT/ pin will assert
low. The maximum value of C_OUT may be approximated
by Equation 1:
C_{OUT_MAX}
=
I_{LIMIT_MIN} × T_{D_FAULT_MIN}
V_{IN_MAX}
Eq. 1
Where: I_{LIMIT_MIN} and T_{D_FAULT_MIN} are the minimum
specified values listed in the Electrical Characteristic
table and V_{IN_MAX} is the maximum input voltage to the
switch.
Current Sensing and Limiting
The MIC2090/MIC2091 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 over-
current threshold. When current limiting is activated in
the -1 version, 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. If the overcurrent fault is large
enough to drop V_OUT below (typically) 1.8V, the internal
MOSFET turns off very quickly (typically 20ns). This
prevents excessive current from flowing through the
device and damaging the internal MOSFET.
The latch-off feature of the -2 version latches the output
off when the output current exceeds the overcurrent
threshold. V_IN or the enable pin must be toggled to reset
the latch.
Enable Input
The EN pin is a TTL logic level compatible input which
turns the internal MOSFET switch on and off. The
FAULT/ pin remains high when the EN pin is pulled low
and the output is turned off. Toggling the enable pin
resets the output after an OGI (output greater than input)
condition occurs. In the -2 version, toggling the enable
pin resets the output after an overcurrent event.
Fault Output
The FAULT/ is an N-channel open-drain output, which is
asserted LOW when the MIC2090/MIC2091 switch
either begins current limiting or enters thermal shutdown.
July 2011
15
M9999-070611-B