Micrel, Inc.
MIC45208 Evaluation Board
August 19, 2015
3
Revision 2.0
SW Node
Test point TP12 (V
SW
) is placed for monitoring the
switching waveform, which is one of the most critical
waveforms for the converter.
Current Limit
The MIC45208 uses the R
DS(ON)
of the low-side MOSFET
and external resistor connected from the ILIM pin to the
SW node to decide the current limit.
Figure 1. MIC45208 Current Limiting Circuit
During each switching cycle of the MIC45208, the inductor
current is sensed by monitoring the low-side MOSFET in
the OFF period. The sensed voltage, V
(ILIM)
, is compared
with the power ground (PGND) after a blanking time of
150ns. In this way the drop voltage over the resistor R15
(V
CL
) is compared with the drop over the bottom FET
generating the short current limit. The small capacitor
(C11) connected from ILIM pin to PGND filters the
switching node ringing during the off-time allowing a better
short-limit measurement. The time constant created by
R15 and C11 should be much less than the minimum off
time.
The V
CL
drop allows programming of short limit through the
value of the resistor (R15) if the absolute value of the
voltage drop on the bottom FET is greater than V
CL
. In that
case the V
(ILIM)
is lower than PGND and a short circuit
event is triggered. A hiccup cycle to treat the short event is
generated. The hiccup sequence including the soft start
reduces the stress on the switching FETs and protects the
load and supply for severe short conditions.
The short circuit current limit can be programmed by using
the following formula:
( )
CL
CL
)
ON
(
DS
PP
L
CLIM
I
V
R
)
5
.
0
I
I
(
R15
+
×
×
D
−
=
Eq. 3
I
CLIM
= Desired current limit
R
DS(ON)
= On-resistance of low-side power MOSFET, 6
mΩ
(typ.)
V
CL
= Current-limit threshold (typical absolute value is
14mV per
Electrical Characteristics
table in MIC45208
datasheet)
I
CL
= Current-limit source current (typical value is 70µA,
per the
Electrical Characteristics
table in MIC45208
datasheet).
ΔI
L(PP)
= Inductor current peak-to-peak, since the inductor
is integrated use Equation 4 to calculate the inductor ripple
current.
The peak-to-peak inductor current ripple is:
L
f
V
)
V
(V
V
I
sw
IN(MAX)
OUT
IN(MAX)
OUT
L(PP)
×
×
−
×
=
D
Eq. 4
The MIC45208 has 0.8µH inductor integrated into the
module. In case of hard short, the short limit is folded
down to allow an indefinite hard short on the output without
any destructive effect. It is mandatory to make sure that
the inductor current used to charge the output capacitance
during soft-start is under the folded short limit; otherwise
the supply will go in hiccup mode and may not be finishing
the soft start successfully.
The MOSFET R
DS(ON)
varies 30 to 40% with temperature.
Therefore, it is recommended to add a 50% margin to I
CLIM
in the above equation to avoid false current limiting due to
increased MOSFET junction temperature rise. With R15 =
1.37k
Ω and C11 = 15pF, the typical output current limit is
16.8A.
