MIC2017 Micrel Semiconductor, MIC2017 Datasheet - Page 15

• Manufacturer Part Number: MIC2017
• Description: (MIC2007 / MIC2017) Adjustable Current Limit Power Distribution Switch
• Manufacturer: Micrel Semiconductor

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Micrel
C
An unavoidable consequence of adding C
capacitance is a reduction in the MIC2008/2018’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 MIC2008/2018 or the system
from a short circuit fault. For this reason, the upper limit
on the value of C
Kickstart (MIC2017)
Kickstart allows brief current surges to pass to the load
before the onset of normal current limiting. This, in turn,
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 MIC2017. The
Kickstart period is governed by an internal timer which
allows current to pass unimpeded 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
MIC2017. This is because 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 4 Amps 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
for as long as the excessive load persists.
Once the MIC2017 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.
October 2005
SLEW
’s effect on I
0.014
0.012
0.008
0.006
0.004
0.002
0.01
14
12
10
vs. External C
0
6
4
2
0
8
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0
SLEW
Typical Turn-on Times
0
LIMIT
0
is 4nF.
Figure 8.
C
0
SLEW
SLEW
0
OUT
0
(nF)
to its programmed limit
Capacitance
0
T
ON
0
T
T
DELAY
RISE
0
SLEW
0
15
Kickstart may be over-ridden by the thermal protection
circuit and if sufficient internal heating occurs, Kickstart
will be terminated and I
load is still present I
ENABLE
Supply Filtering
A 0.1µF to 1µF bypass capacitor positioned close to the
V
practice and required for proper operation of the
MIC2007/2017. This will control supply transients and
ringing. Without a bypass capacitor, large current surges
or an output short may cause sufficient ringing on V
(from supply lead inductance) to cause erratic operation
of the MIC2007/2017’s control circuitry. Good quality,
low ESR capacitors, such as Panasonic’s TE or ECJ
series, are suggested.
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.
Power Dissipation
Power dissipation depends on several factors such as
the load, PCB layout, ambient temperature, and supply
voltage. Calculation of power dissipation can be
accomplished by the following equation:
To relate this to junction temperature, the following
FAULT/
IN
Figure 9. Kickstart Operation with Varying Load
V
I
OUT
OUT
and GND pins of MIC2007/2017 is both good design
P
D
0
=
R
Kickstart
DS(ON)
100
Current Limiting
×
OUT
(
I
200
OUT
OUT
)
I
2
LIMIT
Time (ms)
Load Removed
300
, not I
0. Upon cooling, if the
KICKSTART
400
MIC2007/MIC2017
M9999-102805
500
.
600
IN