sc2441a Semtech Corporation, sc2441a Datasheet - Page 24

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

DCR Current Sensing
Either precision sense resistor or inductor DCR can be
used as the inductor current sensing element.
In Figure 12 SW1 and SW2 represent the MOSFET
switches. CS is the current sense amplifier. I
are the input bias currents of the CS. L
inductor. R
constitute the DCR current sensing network.
Assuming that CS input bias currents are zero and that
R
to the time constant R
inductor DCR, R
state (see Figure 13). The following equations apply:
where, I
peak L
be sensed by monitoring C
so that the
However CS input bias currents are not zero. I
are typically 0.4 A and 40 A respectively (see electrical
characteristics) and can not be ignored.
POWER MANAGEMENT
Applications Information
3
2006 Semtech Corp.
is not used, if the time constant L
V
SW2
SW1
1
IN
current ripple. The inductor current can therefore
O
is the output current and
SC2441A
Figure 12. Current Sensing Circuit.
1
is the DC resistance of L
CS
1
, will be replicated across C
R
I
is between 25% to 33% of the I
L1
I
2
L1
.
V
+
VALLEY
L
-
.
PEAK
C1
1
(t)
2
C
=
1
=
1
I
=
I
, then the voltage across the
CS-
+
I
CS+
voltage. L should be selected
V
O
I
I
L1
O
C1
+
-
(t)
-
D
C
R
R
D
1
I
×
1
3
L
I
R
L
1
1
1
/2
/2
1
I D is the peak-to-
/R
L
1
1
. R
1
1
is made equal
1
is the output
2
in the steady
, R
C
CS+
OUT
CS+
3
and I
and C
O
and I
.
R
LOAD
CS-
CS-
1
24
In Figure 12, R
sensed signals when I
constant resulting from L
Define R
The time constant of the DCR sensing network is:
If
C
I
voltage. To reduce this error, R
Suppose V
F
current limit is set at 28A. The time constant formed by
L
CS+
SW
1
1
V
will be:
and R
C1
=500KHz; L
C1
therefore introduces an offset error to the sensed
×
PEAK
V
=
0
V
Figure 13. Voltage Waveform C
1
EQU
C1
=
L1
is
×
I
VALLEY
:
CS
t
V
,
IN
+
Voltage Drop on R
L1
C1.PEAK
then the peak and valley voltages across
=5V; V
×
V
1
2
R
=
=0.5 H; R
C1.VALLEY
=
Voltage on C
and R
EQU
I
L
R
CS
1
1
+
+
=
×
R
0.25ms
(
3
OUT
R
EQU
D
R
resistive divider attenuates the
2
1
I
3
L
=2.5V; D=50%; I
1
+
1
and its DCR R
+
×
=2m
1
R
R
=
--- V
(
1
1
R
5A
3
--- I
R
=
EQU
2
)
3
C1
+
R
×
×
R
L1
æ
ç
è
R
must be minimized.
2
R
(t)
2
R
(t)R
28A
1
3
I
+
1
CS+
×
)
I
R
L1.VALLEY
R
R
1
×
3
=1 A. The output
3
1
æ
ç
è
SC2441A
+
I
1
I
L1.PEAK
O
×
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1
and R
D
C
-
is:
2
1
I
L1
D
. The time
2
I
ö
÷
ø
L1
=
OUT
1
ö
÷
ø
t
25mV
=20A;
R
1
I
O