MIC20XX_1108 Datasheet, PDF (27/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
MIC20xx Family
The VUVLO comparator uses no hysteresis. This is
because the VUVLO blanking timer prevents any
chattering that might otherwise occur if V_IN varies about
the trigger point. The timer is reset by upward crossings
of the trip point such that V_IN must remain below the trip
point for the full 32ms period for load disengagement to
occur.
In selecting a V_TRIP voltage, the designer is cautioned to
not make this value less than 2.5V. A minimum of 2.5V
is required for the MIC20X6’s internal circuitry to
operate properly. VUVLO trip points below 2.5V will
result in erratic or unpredictable operation.
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 2009X
2018 2019X
Figure 11. Kickstart™
Kickstart™ allows brief current surges to pass to the
load before the onset of normal current limiting, which
permits dynamic loads to draw bursts of energy without
sacrificing system safety.
Functionally, Kickstart™ is a forced override of the
normal current limiting function provided by the switch.
The Kickstart™ period is governed by an internal timer
which allows current to pass up to the secondary
current limit (I_{LIMIT_2nd}) to the load for 128ms and then
normal (primary) current limiting goes into action.
During Kickstart™, a secondary current-limiting circuit
is monitoring output current to prevent damage to the
switch, as a hard short combined with a robust power
supply can result in currents of many tens of amperes.
This secondary current limit is nominally set at 4A and
reacts immediately and independently of the Kickstart™
period. Once the Kickstart™ timer has finished its count
the primary current limiting circuit takes over and holds
I_OUT to its programmed limit for as long as the excessive
load persists.
Once the switch drops out of current limiting the
Kickstart™ timer initiates a lock-out period of 128ms
such that no further bursts of current above the primary
current limit, will be allowed until the lock-out period has
expired.
Kickstart™ may be over-ridden by the thermal
protection circuit and if sufficient internal heating
occurs, Kickstart™ will be terminated and I_OUT Æ 0A.
Upon cooling, if the load is still present I_OUT Æ I_LIMIT, not
I_{LIMIT_2nd}.
Automatic Load Discharge
2003 2004 2005X 2006 2007 2008
2013 2014 2015 2016 2017 2018
Only parts in bold have automatic load discharge.
2009X
2019X
Automatic discharge is a valuable feature when it is
desirable to quickly remove charge from the V_OUT pin.
This allows for a quicker power-down of the load. This
also prevents any charge from being presented to a
device being connected to the V_OUT pin, for example,
USB, 1394, PCMCIA, and CableCARD™.
Automatic discharge is performed by a shunt MOSFET
from V_OUT pin to GND. When the switch is disabled, a
break before make action is performed turning off the
main power MOSFET and then enabling the shunt
MOSFET. The total resistance of the MOSFET and
internal resistances is typically 126Ω.
Supply Filtering
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
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 good quality,
low-ESR capacitors are recommended, preferably
ceramic.
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 0.01μF to 0.1μF are recommended. Again, good
quality, low-ESR capacitors should be chosen.
August 2011
27
M9999-080211-D