SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
parameter sensitive operation. A good engineering In the following design steps, the capacitor CS in the
compromise is to make
current sensing part is commonly selected in the range
∆VCs~ReqδIo.
of 22nF ~ 68nF.
The prerequisite for such relation is the so called time
constant matching condition
L
R eq
≈ RsCs.
Vin
Cin
Q1
V gs1
PN
i L (t )
L
Rs
RL
Rs 1
When Rds1=Rds2, the above relations become
Q2
Cs
equations.
V gs2
For an example of application circuit, L=1.3µH,
vC(t)
RL=1.56mΩ and Rds1=Rds2=8mΩ , the time constant
RsCs should be set as 136µs. If one selects Cs=33 nF,
then Rs=4.12 kΩ.
V be3
Q3
VP N
V be4
Q4
Vo
C out Rload
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
ILMcp
=
75mV ,
R eq
when the load is sourcing current from the converter and
ILMcn
=
110mV
− Req
,
when the load is forcing current back to the input power
source. If R = 9.56mW, then I = 7.8/-11.8A. The circuit
eq
LM
in Figure 16 allows the user to scale the equivalent current
limit with the same Req.
ISEN
+1
-2
Rs 2
R s3
Figure 16. Scaling the equivalent current limit.
a) When the required current limit value ILM is greater
than I , one just needs to remove R , and solve the
LMcp
s3
following equations
(Rs
// Rs1)
Cs
=
L
Req
,
ILMReq
Rs1
Rs + Rs1
=
75mV,
and
Rs2 = Rs // Rs1 .
for
Rs,Rs1 and Rs2.
Note that RS2 is selected as RS//RS1 in order to reduce
the bias current effect of the current amplifier in SC2446.
If the current limit is to be set to ILM = 15A with the existing
power circuit parameter and Cs = 33nF, it is calculated
that Rs2 = 4.12 kΩ, Rs = 7.87 kΩ and Rs1 = 8.66 kΩ.
b) When the required current limit ILM is less than ILMcp,
one just needs to remove Rs1 and solve
2004 Semtech Corp.
20
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