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SC1404 датащи(PDF) 11 Page - Semtech Corporation

номер детали SC1404
подробное описание детали  Mobile Multi-Output PWM Controller with Virtual Current SenseTM
PDF  25 Pages
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производитель  SEMTECH [Semtech Corporation]
домашняя страница  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1404 датащи(HTML) 11 Page - Semtech Corporation

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11
 2004 Semtech Corp.
www.semtech.com
SC1404
POWER MANAGEMENT
Reference Circuit Design
The schematic for the reference circuit is shown on page 20. The
reference circuit is configured as follows:
Switching Regulator 1
Vout1 = 3.3V @ 6A
Switching Regulator 2
Vout2 = 5.0V @ 6A
VL Regulator
Vout3 = 5.0V @ 30mA
12V regulator
Vout4 = 12.0V @ 200mA
Input voltage
Vin = 7 to 21V
Designing the Output Filter
Before calculating the filter inductance and capacitance, an
acceptable inductor ripple current is determined. Ripple current
is usually set at 10% to 20% of the maximum load. However,
increasing the ripple current allows for a smaller inductor and will
also quicken the output transient response. In this example, we
set the ripple current to be 25% of maximum load.
A
A
IO
5
.
1
6
%
25
=
×
=
∆
The inductance is found from ripple current, frequency, input
voltage, and output voltage. Minimum required inductance is
found at maximum Vin, where ripple current is the greatest.
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Io
F
Vin
Vo
Vo
min
L
18
.
6
)
/
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(
=
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×
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=
For the reference design, the Coiltronics DR127-6R8 is used. This
is acceptable for the 3V output, which uses a simple inductor. The
5V inductor must have a 12V winding with a turns ratio of 2.2:1.
For the 5V inductor, the TTI-8215 from Transpower Technologies
is used, which has 5V inductance of 6.4uH.
To specify the output capacitance, the allowable output ripple
voltage must be determined. Output ripple is often specified at
1% of the output voltage. For the 3.3V output, we selected a
maximum ripple voltage of 33mVp-p. The maximum allowable
ESR would then be:
Ω
=
=
∆
∆
=
m
A
mV
I
V
ESR
O
O
MAX
22
5
.
1
/
33
/
Panasonic SP Polymer Aluminum capacitors are a good choice.
For this design, use one 180uF, 4V device, with ESR of 15m
Ω.
The output capacitor must support the inductor RMS ripple current.
To check the actual ripple versus the capacitor’s RMS rating:
A
A
I
I
O
actual
RMS
43
.
0
12
5
.
1
12
_
=
=
∆
=
This is much less than the capacitor’s ripple rating of 3.3A.
Choosing the Main Switching mosfet
The IRF7143 is used in the reference design. Before choosing the
main (high-side) mosfet, we need to check three parameters: volt-
age, power, and current rating.
The maximum drain to source voltage of the mosfet is mainly
determined by the switcher topology. With a buck topology,
V
21
V
V
MAX
_
IN
MAX
_
DS
=
=
The IRF7413 is a 30V device, which allows for 70% derating at
21V operation.
The mosfet power dissipation has three components: conduction
losses, switching losses, and gate drive losses. The conduction
loss is determined using the RMS mosfet current; the equation is
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Startup Sequence Chart
Applications Information



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