Functional Description
The MIC2039 is a high-side MOSF ET power-distribution
switch that provid es increa sed s ystem reliabi lity by using
5% current limit accuracy. The MIC2039 is internally
current limited and has thermal shutdown, which
protects the device and system.
The MIC2039 has a soft-start circuit that minimizes in-
rush current by slowing the turn-on time. Additionally, the
MIC2039 has an optional Kickstart feature, which
momentarily overrides the normal current-limiting
function to allow higher inrush and/or transient currents.
Soft-Start
Soft-start reduces the power-supply input surge current
at startup b y control ling the outpu t voltage ris e tim e. The
input surge appears while the output capacitor is
charged up. A slower output rise time draws a lower
input surge current.
KICKSTART Inrush Overcurrent Filter
The MIC20 39 E Yxx an d MIC203 9F Yxx are equ ipped with
a secondary current limit that allows high inrush current
transients to pass for a set period before the primary
current-limit circuitry becomes ac ti ve. T he FAULT/ status
flag does n ot assert dur ing the Kickstart period ( typica lly
120ms), which elim inates an y false (FAULT/) assertions.
The Kickstart function is active during initial startup or
while operating in steady state.
Input Capacitor
Micrel recommends a 1µF to 10µF ceramic input
capacitor for most applications.
Place the input capacitor on the same side of the board
and next to the MIC20 39 to minimize the voltage ringing
during transient and short-circuit conditions. Using two
vias for each end of the capacitor to connect to the
power and ground plane is also recommended.
Micrel recommends X7R or X5R dielectric ceramic
capacitors because of their temperature performance.
X7R-type capacitors change capacitance by 15% over
their operating temperature range and are the most
stable type of ceramic capacitors. Z5U and Y5V
dielectric capacitors change value by as much as 50%
and 60%, respectively, over their operating temperature
ranges. To use a ceramic chip capacitor with Y5V
dielectric, the value must be much higher than an X7R
ceramic or a tantalum capacitor to ensure the same
capacitance value over the operating temperature range.
Output Capacitor
The outp ut capacitor t ype and pl acement cr iteria are the
same as for the input capacitor. See the “Input
Capacitor” section for a detailed description.
Enable
The MIC2039 offers either an active-high or active-low
enable input (EN) that allows ON/OFF control of the
switch outp ut. The cur rent through the de vice reduces to
near “zero” when the device is shut down, with only
microamperes of leakage current. The EN input can be
directly tied to VIN or driven by a voltage that is equal to
or less than VIN; do not leave this pin floating.
Adjustable Current Limit
The MIC2039 current limit is adjustable from 0.2A to
2.5A by connecting a resistor from the ILIMIT pin to
GND. The following equation determines the resistor:
Eq. 1
where ILIMIT is the typical current limit from the electrical
table. If the output current exceeds the set current limit,
the MIC2039 switch enters constant current limit mode.
The maximum allowable current limit can be less than
the full s pecified an d/or expected current if the MIC2039
is not mounted on a circuit board with sufficiently low
thermal resistance. Table 1 shows resistor values (1%)
for select current limit settings.
ILIMIT 0.2A 0.5A 1.0A 2A 2.5A
RLIMIT 1.45kΩ 576Ω 287Ω 145Ω 115Ω
Table 1. Resistor Selection for Adjustable Current Limit