SC1403 데이터 시트보기 (PDF) - Semtech Corporation
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SC1403 Datasheet PDF : 31 Pages
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SC1403
POWER MANAGEMENT
Functional Information (Cont.)
Startup Sequence Chart
SEQ
ON3
ON5
RESET
DESCRIPTION
REF
LOW
LOW
Follows 3.3V SMPS.
Independant start control mode. Both SMPSs off.
REF
REF
REF
GND
GND
VL
VL
LOW
HIGH
HIGH
LOW
HIGH
LOW
HIGH
HIGH
LOW
HIGH
X
HIGH/LOW
X
HIGH/LOW
Low.
Follows 3.3V SMPS.
Follows 3.3V SMPS.
Low.
High after both outputs are
in regulation.
Low.
High after both outputs are
in regulation.
5V SMPS ON, 3.3V SMPS OFF.
3.3V SMPS ON, 5V SMPS OFF.
Both SMPSs on.
Both SMPSs off.
5V starts when ON3 goes high. If ON5 = HIGH,
3V is on. If ON5 = LOW, 3V is off.
Both SMPSs off.
3V starts when ON3 goes high. If ON5 = HIGH,
5V is on. If ON5 = LOW, 5V is off.
Applications Information
Reference Circuit Design
The schematic for the reference circuit is shown on page 27. The
reference circuit is configured as follows:
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:
ESR MAX = ∆VO / ∆IO = 33mV /1.5A = 22mΩ
Switching Regulator 1
Switching Regulator 2
Linear Regulator
Input voltage
Vout1 = 3.3V @ 6A
Vout2 = 5.0V @ 6A
Vout3 = 5.0V @ 50mA
Vin = 7 to 21V
Designing the Output Filter
Before calculating the filter inductance and capacitance, an
acceptable inductor ripple current is determined. Maximum
allowable ripple depends on the transient requirements. 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.
∆IO = 25 % × 6 A = 1.5A
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.
L min
=
Vo
×
(1 − Vo / Vin
F × ∆ Io
)
=
6.18 uH
The next standard value is 6.8uH. For the reference design, the
Coiltronics DR127-6R8 is used.
Panasonic SP Polymer Aluminum capacitors are a good choice.
For this design, use one 180uF, 4V device, with ESR of 15mΩ.
The output capacitor must support the inductor RMS ripple current.
To check the actual ripple versus the capacitor’s RMS rating:
I = RMS_ actual ∆IO = 1.5A = 0.43A
12 12
This is much less than the capacitor’s ripple rating of 3.3A.
Choosing Current Sense Components
Since the SC1403 implements Virtual Current SenseTM, current
sensing is not required for the control loop. But it is required for
cycle-by-cycle current limit and for startup. Cycle-by-cycle current
limit is reached when the voltage of CSH-CSL exceeds 55mV nomi-
nal. Depending on the system requirement, this current limit can
vary, but it is typically 10% to 30% higher than the maximum load.
This design uses the DC resistance of the inductor as a current
sense element, which eliminates the cost and space required for
a separate current sense resistor. Below is a typical DCR applica-
tion circuit. The inductor is shown along with it’s wiring resistance
RL. In place of the current sense resistor are C, R2, and R1, which
are connected across the inductor terminals.
To specify the output capacitance, the allowable output ripple
2004 Semtech Corp.
11
United States Patent No. 6,377,032
www.semtech.com
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