MIC20XX_1108 Datasheet, PDF (25/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
Normalized Output Current
vs. Output Voltage (2.5V)
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.5 1.0 1.5 2.0 2.5 3.0
OUTPUT VOLTAGE (V)
Figure 7. Normalized Output Current vs. Output Voltage
C_SLEW
2003 2004 2005X 2006 2007
2013 2014 2015 2016 2017
Only parts in bold have CSLEW pin.
(Not available in 5-pin SOT-23 packages).
2008
2018
2009X
2019X
The C_SLEW pin is provided to increase control of the
output voltage ramp at turn-on. This input allows
designers the option of decreasing the output’s slew
rate (slowing the voltage rise) by adding an external
capacitance between the C_SLEW and V_IN pins. This
capacitance slows the rate at which the pass FET gate
voltage increases and thus, slows both the response to
an Enable command as well as V_OUT’s ascent to its final
value.
Figure 8 illustrates effect of C_SLEW on turn-on delay and
output rise time.
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
MIC20xx Family
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.
Variable Undervoltage Lock Out (VUVLO)
2003 2004 2005X 2006 2007 2008
2013 2014 2015 2016 2017 2018
Only parts in bold have VUVLO pin and functionality.
2009X
2019X
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 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.
August 2011
25
M9999-080211-D