SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441ITSTRT
Semtech Corporation
SC2441ITSTRT Datasheet PDF : 35 Pages
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SC2441
POWER MANAGEMENT
Applications Information (Cont.)
Given the input voltage Vin, the output voltage Vo3 and
the load current Io3, the following circuit parameters are
to be determined.
Inductor Selection
For a specified inductor current ripple (peak-to-peak) δ3,
the inductor value is
L3
=
Vin
fs3 δ 3Io3
(1−
Vin
Vo3
) Vin
Vo3
.
If Vin = 3.3V, Vo3 = 5V and Io3 = 1A withδ3 = 0.3 and fs3 =
1MHz, then, L3 = 2.5µH.
For operation above 1MHz, Ferrite core material should
be used for lower core losses and better circuit efficiency.
The peak inductor current is
(1+0.5*δ3)Io3.
The saturation current rating of the selected inductor
should be at least 1.2~1.5 times of the calculated peak
current value. In the example above, the peak inductor
current is 1.74A.
Maximum Output Current
Figure 4 shows that the boost switch current is sensed
with an internal sense resistor Rs. With a current limit
threshold voltage of 14mV, the maximum output current
is
Io3,max
=
(1− D3 )14mV .
Rs (1+ δ3 2 )
The boost switch current limit is internally set at 1.7A.
Voltage Feedback
With the internal reference voltage Vref3=1.25V, the voltage
feedback gain is
h3
=
Vref 3
Vo3
1.25
=5
= 0.25.
The values of the feedback resistors can be determined
from the equation in “Setting the Output Voltage” section.
Loop Compensation
Capacitor Selection
Input capacitor: The input capacitance should be large
such that the input transients due to both the step-up
and the step-down converters do not trip the UVLO
threshold 1.59V. Since the SC2441 controls a 2-phase
low input voltage step-down converter, the input
capacitance is sized to handle the input ripple current of
the buck converter. This is usually sufficient for the auxiliary
boost converter because the input current in a boost
converter is continuous. A small (< 1uF) ceramic bypass
capacitor can be soldered close to the PVIN pin for high
frequency filtering.
A simple small signal model for current-mode boost
converter in continuous-conduction mode is shown in
Figure 17.
(I
−
D3
)I
−
(I
−
SL 3
D3 )2RO3
Output capacitor: Unlike buck converter, pulse current is
delivered to the output of a boost converter. To reduce the
output ripple voltage, low ESR capacitors should be used.
The output capacitor should also be able handle the output
ripple current. The SC2441 is designed to use multi-layer
ceramic capacitor as the sole output capacitor.
Figure 17. Small signal model of Boost converter.
2005 Semtech Corp.
25
www.semtech.com
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