SC2446A Semtech Corporation, SC2446A Datasheet - Page 15

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

It is noted that the DC value of V
value of L, R
load current information is needed (such as in average
current mode control), this current sensing method is
effective without time constant matching requirement.
In the current mode control as implemented in SC2446A,
the voltage ripple on C
fact, the AC voltage ripple peak-to-peak value of V
(denoted as V
of the PWM operation. In general, smaller V
lower signal-to-noise ratio and more noise sensitive
operation. Larger V
parameter sensitive operation. A good engineering
compromise is to make
The pre-requisite for such relation is the so called time
constant matching condition
For an example of application circuit, L = 1µH,
RL = 1.8m
555.6µs. If one selects C
Scaling the Current Limit
Over-current is handled differently in the SC2446A
depending on the direction of the inductor current. If the
differential sense voltage between CS+ and CS- exceeds
+50mV, the top MOSFET will be turned off and the bottom
MOSFET will be turned on 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. If the voltage between
CS1+ and CS- falls below -75mV, 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 11, the equivalent inductor current
limits are set according to
POWER MANAGEMENT
Application Information (Cont.)
2005 Semtech Corp.
s
and C
the time constant R
Cs
) directly effects the signal-to-noise ratio
s
. This means that, if only the average
Cs
I
leads to more circuit (power stage)
LMcp
R
s
L
is critical for PWM operation. In
V
L
Cs
s
~ R
= 33nF, then R
50
R
R
s
mV
C
L
L
s
Cs
.
I
,
o
is independent of the
.
s
C
s
should be set as
s
= 16.9 k .
Cs
leads to
Cs
15
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 11 allows the user to scale the equivalent
current limit with the same RL.
In the following design steps, the capacitor C
current sensing part is commonly selected in the range
of 22nF ~ 100nF.
a) When the required current limit value I
than I
following equations
and
for
Vin
LMcp
Figure 11. Scaling the equivalent current limit.
, one just needs to remove R
Cin
L
= 1.8m , then I
R
R
Vgs1
Vgs2
ISEN
s
s
2
PN
R ,
I
+
-
LM
1 s
1
2
R
R (
R
I
Q1
Q2
and
LMcn
s
L
s
//
R
//
iL(t)
R
R
s
R
R
1 s
1 s
s
1 s
.
2
R
)
L
Rs
.
LM
C
75
1 s
Rs3
s
= 27.8 / -41.7A. The circuit
R
mV
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L
RL
50
Rs1
vC(t)
Rs2
R
Cs
L
L
mV
,
,
,
SC2446A
s3
Cout
, and solve the
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LM
Rload
is greater
S
Vo
in the