SC2446 Semtech Corporation, SC2446 Datasheet - Page 20

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

parameter sensitive operation. A good engineering
compromise is to make
The prerequisite for such relation is the so called time
constant matching condition
When Rds1=Rds2, the above relations become
equations.
For an example of application circuit, L=1.3µH,
RL=1.56m
R
then R
Scaling the Current Limit
Over-current is handled differently in the SC2446
depending on the direction of the inductor current. If the
differential sense voltage between CS+ and CS- exceeds
+75mV, the top MOSFET will be turned off and the bottom
MOSFET will be turned on to limit the inductor current.
This +75mV is the cycle-by-cycle peak current limit when
the load is drawing current from the converter. There is
no cycle-by-cycle current limit when the inductor current
flows in the reverse direction. If the voltage between
CS1+ and CS- falls below -113mV, the controller will
undergo overload shutdown and time-out with both the
top and the bottom MOSFETs shut off. (See the section
Overload Protection and Hiccup).
In the circuit of Figure 14, the equivalent inductor current
limits are set according to
when the load is sourcing current from the converter and
when the load is forcing current back to the input power
source. If R
in Figure 16 allows the user to scale the equivalent current
limit with the same R
POWER MANAGEMENT
Application Information (Cont.)
s
C
2004 Semtech Corp.
s
should be set as 136µs. If one selects C
s
=4.12 k .
eq
= 9.56mW, then I
and Rds1=Rds2=8m
I
LMcn
eq
I
LMcp
R
.
L
eq
V
Cs
~R
75
110
R
R
R
s
eq
mV
LM
C
eq
mV
eq
s
= 7.8/-11.8A. The circuit
I
.
o
,
.
,
, the time constant
s
=33 nF,
20
In the following design steps, the capacitor C
current sensing part is commonly selected in the range
of 22nF ~ 68nF.
a) When the required current limit value I
than I
following equations
and
for
Note that R
the bias current effect of the current amplifier in SC2446.
If the current limit is to be set to I
power circuit parameter and C
that R
b) When the required current limit I
one just needs to remove R
Vin
Cin
LMcp
s2
Figure 16. Scaling the equivalent current limit.
= 4.12 k , R
, one just needs to remove R
V be3
V be4
V gs1
V gs2
IS E N
S2
VP N
P N
R
is selected as R
R
+
-
s
s
1
2
Q3
Q4
2
R ,
I
Q1
Q2
LM
1 s
R (
R
R
iL(t)
and
s
eq
s
s
//
//
= 7.87 k
R
R
R
s
R
1 s
R
1 s
L
Rs
s
1 s
.
)
2
R s3
R
.
s1
C
1 s
s
and solve
S
RL
s
//R
Rs 1
vC(t)
Rs 2
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LM
Cs
= 33nF, it is calculated
75
R
and R
L
= 15A with the existing
eq
S1
mV
,
LM
in order to reduce
,
C out
is less than I
s3
s1
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, and solve the
SC2446
= 8.66 k .
Rload
LM
is greater
S
Vo
in the
LMcp
,