SC2446AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2446AEVB Datasheet PDF : 27 Pages
| |||
SC2446A
POWER MANAGEMENT
Application Information (Cont.)
The power dissipated in the input capacitors is then
PCin = ICin2Resr.
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10oC of
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
∆v ESR
= Resr (1+
δ
2
)Io
.
The peak-to-peak input voltage ripple due to the capacitor
is
∆v C
≈
DIo
Cinfs
,
From these two expressions, CIN can be found to meet
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.
Let the duty ratio and output current of Channel 1 and
Channel 2 be D1, D2 and Io1, Io2, respectively.
If D1<0.5 and D2<0.5, then
ICin ≈ D1Io12 + D2Io22 .
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 sense
DS(ON)
The basic arrangement of the inductive current sense is
shown in Figure 10.
Where, RL is the equivalent series resistance of the output
inductor. The added Rs and Cs form a RC branch for
inductor current sensing.
Vin
Ripple cancellation effect of interleaving allows the use
of smaller input capacitors. When converter outputs are
connected in parallel and interleaved, smaller inductors
Cin
and capacitors can be used for each channel. The total
output ripple-voltage remains unchanged. Smaller
inductors speeds up output load transient.
Q1
iL(t)
L
Rs
Q2
RL
Cs
vC(t)
Vo
Cout Rload
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.
Figure 10. The basic structure of inductive current
sense.
In steady state, the DC value,
VCS = RL IO
2005 Semtech Corp.
14
www.semtech.com
Share Link: 