MIC20XX Datasheet, PDF (25/29 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
MIC20xx Family
Typical Turn-on Times
vs. External C Capacitance
SLEW
0.014
T_ON
0.01122
0.0110
T_DELAY
0.0088
0.0066
0.0044
0.0022
T_RISE
0
00 00.5 10 10.5 ₀₂ 20.5 ₀₃ 30.5 ₀₄ 40.5
C_SLEW (nF)
Figure 8. C_SLEW vs. Turn-On, Delay and Rise TImes
C_SLEW’s effect on I_LIMIT
An unavoidable consequence of adding C_SLEW
capacitance is a reduction in the MIC20X5 – 20X8’s
ability to quickly limit current transients or surges. A
sufficiently large capacitance can prevent both the
primary and secondary current limits from acting in time
to prevent damage to the MIC20X5 – 20X8 or the
system from a short circuit fault. For this reason, the
upper limit on the value of C_SLEW is 4nF.
programmed current limit. When the weaker, standby
supply is in operation, the MIC20X6 monitors V_IN and will
shut off its output should V_IN dip below a predetermined
value. This predetermined voltage is user programmable
and set by the selection of the resistor divider driving the
VUVLO pin.
To prevent false triggering of the VUVLO feature, the
MIC20X6 includes a delay timer to blank out momentary
excursions below the VUVLO trip point. If V_IN stays
below the VUVLO trip point for longer than 32ms
(typical), then the load is disengaged and the MIC20X6
will wait 128ms before reapplying power to the load. If
V_IN remains below the VUVLO trip point, then the load
will be powered for the 32ms blanking period and then
again disengaged. This is illustrated in the scope plot
below. If V_IN remains above the VUVLO trip point
MIC20X6 resumes normal operation.
Variable Under Voltage Lock Out (VUVLO)
2003 2004 2005X 2006 2007 2008 2009X
2013 2014 2015 2016 2017 2018 2019X
Only parts in bold have VUVLO pin and functionality.
Power conscious systems, such as those implementing
ACPI, will remain active even in their low power states
and may require the support of external devices through
both phases of operation. Under these conditions, the
current allowed these external devices may vary
according to the system’s operating state and as such
require dual current limits on their peripheral ports. The
MIC20X6 is designed for systems demanding two
primary current limiting levels but without the use of a
control signal to select between current limits.
To better understand how the MIC20X6 provides this,
imagine a system whose main power supply supports
heavy loads during normal operation, but in sleep mode
is reduced to only few hundred milliamps of output
current. In addition, this system has several USB ports
which must remain active during sleep. During normal
operation, each port can support a 500mA peripheral,
but in sleep mode their combined output current is
limited to what the power supply can deliver minus
whatever the system itself is drawing.
If a peripheral device is plugged in which demands more
current than is available, the system power supply will
sag, or crash. The MIC20X6 prevents this by monitoring
both the load current and V_IN. During normal operation,
when the power supply can source plenty of current, the
MIC20X6 will support any load up to its factory
Figure 9. VUVLO Operation
VUVLO and Kickstart™ operate independently in the
MIC2016. If the high current surge allowed by
Kickstart™ causes V_IN to dip below the VUVLO trip point
for more than 32ms, VUVLO will disengage the load
even though the Kickstart™ timer has not timed out.
I_{IN_LOAD}
Input
Supply
+
VIN
VOUT
R1
MIC20X6
+
VUVLO
R2
Figure 10. VUVLO Application Circuit
Calculating VUVLO Resistor Divider Values
The VUVLO feature is designed to keep the internal
switch off until the voltage on the VUVLO pin is greater
than 0.25V. A resistor divider network connected to the
VUVLO and VIN pins is used to set the input trip voltage
V_TRIP, see Figure 10. The value of R2 is chosen to
minimize the load on the input supply I_DIV and the value
of R1 sets the trip voltage V_TRIP.
February 2011
25
M9999-020311-D