SC1403(2002) データシートの表示（PDF） - Semtech Corporation
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SC1403 Datasheet PDF : 30 Pages
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SC1403
POWER MANAGEMENT
Functional Information (Cont.)
SC1403 Startup Sequence Chart
PRELIMINARY
SEQ
REF
REF
REF
REF
GND
GND
ON3
LOW
LOW
HIGH
HIGH
LOW
HIGH
ON5
LOW
HIGH
LOW
HIGH
X
HIGH/LOW
VL
LOW
X
VL
HIGH
HIGH/LOW
RESET
Follows 3.3V SMPS.
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.
DESCRIPTION
Independant start control mode. Both SMPSs off.
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 ON3 = LOW, 3V is off.
Both SMPSs off.
3V starts when ON3 goes high. If ON5 = HIGH,
5V is on. If ON3 = LOW, 5V is off.
Applications Information
Design Guidelines
The schematic for the reference circuit is shown on page 24. The
reference circuit is configured as follows:
Switching Regulator 1
Switching Regulator 2
Linear Regulator
Vout1 = 3.3V @ 6A
Vout2 = 5.0V @ 6A
Vout3 = 5.0V @ 50mA
Designing the Output Filter
Before calculating the output filter inductance and output
capacitance, an acceptable amount of output ripple current is to
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 at 10 to 20% of the
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
0.015Ω
=
2.2A
Checking to see if the maximum RMS current can be met by the
SP cap.
I1
=
−
∆IO
2
IRMS =
I12 + I1 ⋅I2 + I22
3
I2
=
+
∆IO
2
Irms=0.635 A << Irms_rated=3.0A
The output inductance can now be found by:
LO
=
(VIN _
NOM − VO) ⋅DNOM ⋅ TS
∆IO
2002 Semtech Corp.
11
www.semtech.com
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