MIC20XX Datasheet, PDF (26/29 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
MIC20xx Family
The value of R2 is calculated using:
R2 = ^VVUVLO
I_DIV
The vale of R1 is calculated using:
R1
=
R2
×
⎜⎜⎝⎛
V_TRIP
V_VUVLO
− 1_{⎟⎟⎠⎞}
Where for both equations:
V_VUVLO = 0.25V
When working with large value resistors, a small amount
of leakage current from the VUVLO terminal can cause
voltage offsets that degrade system accuracy.
Therefore, the maximum recommended resistor value
for R2 is 100kΩ.
Using the divider loading current I_DIV of 100uA, the value
of R2 can be estimated by:
R2 = ^0.25V = 2.5kΩ
100µA
Now the value of R1 can be calculated by:
R1 = 2.5kΩ × ⎜⎛ ^4.75V − 1⎟⎞ = 45k
⎝ ^0.25V ⎠
Where:
V_TRIP = 4.75V (for a 5V supply)
V_VUVLO = 0.25V
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 2008 2009X
2013 2014 2015 2016 2017 2018 2019X
Only parts in bold have Kickstart™.
(Not available in 5-pin SOT-23 packages).
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}.
Figure 11. Kickstart™
Automatic Load Discharge
2003 2004 2005X 2006 2007 2008 2009X
2013 2014 2015 2016 2017 2018 2019X
Only parts in bold have automatic load discharge.
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
February 2011
26
M9999-020311-D