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MIC24420 Datasheet(PDF) 17 Page - Micrel Semiconductor

Part No. MIC24420
Description  2.5A Dual Output PWM Synchronous Buck Regulator IC
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Manufacturer  MICREL [Micrel Semiconductor]
Direct Link  http://www.micrel.com
Logo MICREL - Micrel Semiconductor

MIC24420 Datasheet(HTML) 17 Page - Micrel Semiconductor

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Micrel, Inc.
MIC24420/MIC24421
June 2012
17
M9999-062012-C
C
S
SS
OUT
OUT
I
I
V
C
/
>
SS
C
Where
IS/C is the short circuit, fold-back current limit.
CSS is the capacitor connected to EN/DLY pin
ISS is the EN/DLY pull up current.
Current Limit Resistor
The current limit circuit responds to the peak inductor
current flowing through the low-side FET. Calculating the
current setting resistor RCS should take into account the
peak inductor current and the blanking delay of
approximately 100ns.
Figure 9. Overcurrent waveform
Figure 9 shows the low-side MOSFET current waveform.
Peak current is measured after a small delay. The
equations used to calculate the current limit resistor
value are shown below:
2
OUT
PK
I
I
I
PP
=
L
t
V
I
I
DLY
OUT
PK
OC
=
CS
ON
OC
CS
I
RDS
I
R
=
Where:
IOC is the current limit set point
L = inductor value
tDLY = Current limit blanking time ~ 100ns
ICS is the overcurrent pin sense current (200µA nominal)
RDS(ON) is the on resistance of the low-side MOSFET
Short Circuit Protection
It is recommended that a fold-back current characteristic
be implemented to protect both external and internal
MOSFETs during short circuit (S/C) events. This can be
achieved by the addition of one additional resistor RFBK
(R14 & R19 on the evaluation board) from VOUT to the
CS pin.
Figure 9a. Short Circuit Protection
Current limit will occur at:
⎟⎟
⎜⎜
+
=
OFF
CS
FBK
CS
OUT
CS
CS
ON
DS
OC
V
R
R
V
R
I
R
I
_
)
(
1
Where VCS_OFF is the CS comparator offset voltage.
For simplicity, assuming VCS_OFF is 0V, we can set IS/C
(current limit when VOUT = 0V) to be half IOC (current limit
when VOUT = nominal):
CS
OUT
FBK
CS
ON
DS
OC
CS
I
V
R
I
R
I
R
=
=
2
)
(
To determine worst case values, one must take into
account VCS offset voltage, ICS range and the range of
values for RDS(ON) over the operating temperature range.
Some typical example values for a 30m
Ω MOSFET:
VOUT
IOC
IS/C
RCS
RFBK
3.3A
1.7
249
24.9k
5
4.3A
1.7
249
16k
3.3A
1.7
249
16.5k
3.3
4.3A
1.7
249
10.5k
3.3A
1.7
249
5.1k
1.2
4.3A
1.7
249
3.83k
Due to the leading edge blanking, a 100ns slew rate for
the CS pin can be applied without interfering with current
limit operation. Limiting the CS pin’s slew rate will help to
prevent false triggering. A C·R product of at least 20ns
should be used.


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