SC2447 Semtech Corporation, SC2447 Datasheet - Page 20

• Manufacturer Part Number: SC2447
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

average current mode control), this current sensing
method is sufficient without time constant matching
requirement.
In the current mode control as implemented in SC2447,
voltage ripple on C
fact, the V
operation. In general, smaller V
to-noise ratio and more noise sensitive operation. Larger
sensitive operation. A good engineering compromise is:
It is necessary to match the following time constants to
equalize the ripple.
For example, L = 1µH and R
R
then R
Scaling the Current Limit
Over-current is handled differently in the SC2447
depending on the direction of the inductor current. If
the differential sense voltage between CS+ and CS-
exceeds +50mV, the PWM signal (GDH) will go low. The
MOSFET driver will turn off the top MOSFET and turn on
the bottom MOSFET, to limit the inductor current. This
+50mV 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.
In the circuit of Figure 13, the equivalent inductor current
limit is set according to
when the load is sourcing current from the converter. If
R
allows the user to scale the equivalent current limit with
the same R
C
POWER MANAGEMENT
Application Information (Cont.)
V
s
L
S
V
C
= 1.8m , then I
in the current sensing network is usually in the range
2006 Semtech Corp.
Cs
Cs
s
) directly affects the signal-to-noise ratio of the PWM
should be set to 555.6µs. If one selects C
leads to more circuit (power stage) parameter
s
= 16.9 k .
Cs
L
.
AC peak-to-peak voltage ripple (denoted as
LMcp
s
is required for PWM operation. In
I
LMcp
= 27.8A. The circuit in Figure 14
R
L
V
L
Cs
≈
=
~ R
L
50
R
= 1.8m
s
R
mV
C
L
L
s
Cs
.
I
,
o
leads to lower signal-
.
the time constant
s
= 33nF,
20
of 22nF ~ 330nF.
a) When the required current limit I
R
for R
and
Note that R
of the bias current of the current amplifier in SC2447.
b) When the required current limit I
remove R
and R
R
s3
s2
Figure 14. Scaling the Equivalent Current Limit
is not needed and solve the following three equations
is then calculated from
s
, R
s3
:
Vin
s1
s1
, and R
S2
Cin
and solve the following two equations for R
is made equal to R
R =
s
Vgs1
Vgs2
ISEN
2
s2
I
I
PN
LM
:
LM
+
-
R
(
1
2
R
R
R
s
Q1
Q2
R
L
s
L
//
s
//
R
+
R
2
iL(t)
R
s
R
s
=
C
R
s
R
R
1
+
s
s
1
s
s
s
.
R
3
)
1
R
=
L
Rs
R
V
s
C
3
s
Rs3
1
s
O
R
3
s
−
L
R
=
L
=
=
R
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RL
s
,
50
Rs1
vC(t)
Rs2
50
R
S
s
Cs
L
//R
.
L
mV
LM
mV
,
LM
is higher than I
S1
,
,
Cout
is less than I
to reduce effect
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SC2447
Rload
Vo
LMcp
LMcp
,
,
s