SC1402(2001) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1402
(Rev.:2001)
(Rev.:2001)
Semtech Corporation
SC1402 Datasheet PDF : 18 Pages
| |||
SC1402
POWER MANAGEMENT
SC1402 Startup Sequence Chart
SEQ
REF
RUN/ON3 TIME/ON5
LOW
LOW
RESET
Follows 3.3V SMPS.
REF
REF
REF
GND
GND
VL
VL
LOW
HIGH
HIGH
LOW
HIGH
LOW
HIGH
HIGH
LOW
HIGH
TIMING CAP
TIMING CAP
TIMING CAP
TIMING CAP
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 on rising edge of RUN/ON3, 3V
starts when TIME/ON5 > REF.
Both SMPSs off.
3.3V starts on rising edge of RUN/ON3, 5V
starts when TIME/ON5 > REF.
Application Information
Introduction
The SC1402 is a versatile dual switching regulator adjustable
from 2.5V to 5.5V with fixed 5V and 3.3V modes. In addition,
there are two on-chip 12V & 5V linear regulators capable of
supplying 120mA & 50mA of output current, respectively. The
SC1402 is designed for notebook applications but has
applications anywhere high efficiency, small size and low cost
are required.
The Semtech SC1402 EVAL board consists of a 3.3V, 3A
switcher, a 5.0V, 3A switcher, an onboard 12V linear regulator.
A 15V flyback supply developed off the 5V SMPS inductor
delivers the input voltage to the onboard 12V linear regulator.
Design Guidelines
So once the ripple voltage specification is selected, the
capacitor ESR is chosen usually based on capacitor cost and
size constraints. This then sets the maximum output ripple
current through the capacitor and inductor.
For the EVAL board 3.3V switcher, we selected a maximum
ripple voltage of 50mV. Choosing two 330uF, 6.3V tantalum
capacitors C21 & C22, each having an ESR of 100mW, their
combined ESR equaling 50mW, sets the maximum ripple
current as follows:
Δ IO
=
ΔVO
ESR
ΔIO
=
0.05
0.05
= 1Amp
Be sure that the two output capacitors can handle this ripple
current. Ripple current specifications are found in the capacitor
data sheet for high quality capacitors intended for use in
switching power supplies.
The schematic for the EVAL board is shown on page 18. The
EVAL board is configured as follows:
Switching Regulator 1
Switching Regulator 2
Linear Regulator 1
Vout1 = 3.3V, 3A
Vout2 = 5.0V, 3A
Vout3 = 12V, 120mA
Linear Regulator 2
Vout4 = 5V, 50mA
The inductance can now be found:
L1 = (VINMAX
− VO ) • D • t
ΔIO
These inductance and duty cycle equations exclude any
resistive drops due to winding resistance and MOSFET on
resistance.
Designing the Output Filter
Where:
Before calculating the output filter inductance and output
capacitor, an acceptable amount of output ripple current is to
be determined. The ESR of the output capacitor multiplied by
the ripple current sets the maximum allowable ripple voltage.
For f = 300KHz, t = 1/f
D=
VO
VIN
;L1 =
(28
− 3.3)
•
3.3
28
1
• 3.33
• 10 −6
2001 Semtech Corp.
12
www.semtech.com
Share Link: 