SC2446A Semtech Corporation, SC2446A Datasheet - Page 14

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

The power dissipated in the input capacitors is then
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10
temperature rise. Many manufacturers specify the
maximum allowable ripple current (ARMS) rating of the
capacitor at a given ripple frequency and ambient
temperature. The input capacitance should be high
enough to handle the ripple current. For higher power
applications, multiple capacitors are placed in parallel to
increase the ripple current handling capability.
Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is
The peak-to-peak input voltage ripple due to the capacitor
is
From these two expressions, C
the input voltage ripple specification. In a multi-phase
converter, channel interleaving can be used to reduce
ripple. The two step-down channels of the SC2446A
operate at 180 degrees from each other. If both step-
down channels in the SC2446A are connected in parallel,
both the input and the output RMS currents will be
reduced.
Ripple cancellation effect of interleaving allows the use
of smaller input capacitors. When converter outputs are
connected in parallel and interleaved, smaller inductors
and capacitors can be used for each channel. The total
output ripple-voltage remains unchanged. Smaller
inductors speeds up output load transient.
When two channels with a common input are interleaved,
the total DC input current is simply the sum of the
individual DC input currents. The combined input current
waveform depends on duty ratio and the output current
waveform. Assuming that the output current ripple is
small, the following formula can be used to estimate the
RMS value of the ripple current in the input capacitor.
POWER MANAGEMENT
Application Information (Cont.)
2005 Semtech Corp.
v
ESR
P
Cin
R
v
= I
C
esr
1 (
Cin
C
2
DI
R
in
2
o
f
esr
IN
s
)
. I
.
,
o
can be found to meet
o
C of
14
Let the duty ratio and output current of Channel 1 and
Channel 2 be D
If D
Choosing Power MOSFETs
The power devices with integrated gate drivers such as
PIP212, R2J20601NP, PIP202, PIP201 and IP2001,
IP2002 are suitable for SC2446A application.
Current Sensing
Inductor current sensing is required for the current-mode
control. Although the inductor current can be sensed with
a precision resistor in series with the inductor, the lossless
inductive current sense technique is used in the
SC2446A. This technique has the advantages of
1) lossless current sensing,
2) lower cost compared to resistive sense
3) more accurate compared to the R
The basic arrangement of the inductive current sense is
shown in Figure 10.
Where, R
inductor. The added R
inductor current sensing.
Figure 10. The basic structure of inductive current
sense.
In steady state, the DC value,
I
Cin
Vin
1
<0.5 and D
D
1
I
L
o
Cin
1
is the equivalent series resistance of the output
2
D
2
I
1
2
o
, D
<0.5, then
2
2
.
Q1
Q2
2
and I
iL(t)
V
s
CS
and C
o1
= R
L
Rs
, I
o2
L
, respectively.
I
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s
O
RL
Cs
vC(t)
form a RC branch for
DS(ON)
SC2446A
www.semtech.com
Cout
sense
Rload
Vo