SC1404(2003) Просмотр технического описания (PDF) - Semtech Corporation
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SC1404 Datasheet PDF : 27 Pages
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SC1404
POWER MANAGEMENT
Functional Information
SC1404 Startup Sequence Chart
SEQ
REF
REF
REF
REF
GND
GND
ON3
LOW
LOW
HIGH
HIGH
LOW
HIGH
VL
LOW
VL
HIGH
ON5
RESET#
DESCRIPTION
LOW
Follows 3.3V SMPS.
Independant start control mode. Both SMPSs off.
HIGH
Low.
5V SMPS ON, 3.3V SMPS OFF.
LOW
Follows 3.3V SMPS.
3.3V SMPS ON, 5V SMPS OFF.
HIGH
Follows 3.3V SMPS.
Both SMPSs on.
X
Low.
Both SMPSs off.
HIGH/LOW High after both outputs are in 5V starts when ON3 goes high. If ON5= HIGH, 3V is
regulation.
on. IF ON5 = LOW, 3V is off.
X
Low.
Both SMPSs off.
HIGH/LOW High after both outputs are in 3V starts when ON3 goes high. If ON5 = HIGH, 5V is
regulation.
on. IF ON5 = LOW, 5V is off.
Applications Information
Reference Circuit Design
Introduction
The SC1404 is a versatile dual switching regulator with fixed 5V
and 3.3V outputs . In addition, there is an on-chip 5V linear regulator
capable of supplying 50mA output current and a 12V linear regulator
able to provide 200mA. The SC1404 is designed for notebook
applications but has is suited to applications where high efficiency,
small package, and low cost are required.
Design Guidelines
maximum load. However, in order to speed up the output transient
response, ripple current can be much higher. In this design, we are
going to set the ripple current to be 40% of maximum load. So
once the ripple voltage specification is determined, the capacitor
ESR is chosen. The output ripple voltage is usually specified at +/
- 1% of the output voltage.
For the reference circuit 3.3V switcher, we selected a maximum
ripple voltage of 33mV. Choosing one 180uF, 4V Panasonic SP
Polymer Aluminum Electrolytic Cap, of which ESR is 15 mΩ , sets
the maximum ripple current as follows:
∆IO = ∆VO _ MAX
ESR
∆IO = 0.033V = 2.2A
0.015Ω
The schematic for the reference circuit is shown on page 22. The
reference circuit is configured as follows:
Switching Regulator 1
Switching Regulator 2
Linear Regulator 1
Linear Regulator 2
Vout1 = 3.3V @ 6A
Vout2 = 5.0V @ 6A
Vout3 = 12V, 200mA
Vout3 = 5.0V @ 50mA
Checking to see if the maximum RMS current can be met by the
SP cap.
IRMS = I12 + I1 ⋅I2 + I22
3
I1 = − ∆IO
2
I2 = + ∆IO
2
Designing the Output Filter
Irms=0.635 A << Irms_rated=3.0A
Before calculating the output filter inductance and output
capacitance, an acceptable amount of output ripple current must
be determined. The maximum allowable ripple current depends
on the transient requirement of the power supply. Under normal
situation, the ripple current is usually set around 10 to 20% of the
The output inductance can now be found by:
LO = (VIN _ NOM − VO) ⋅DNOM ⋅ TS
∆IO
2003 Semtech Corp.
11
www.semtech.com
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