SC1409 Semtech Corporation, SC1409 Datasheet - Page 7

• Manufacturer Part Number: SC1409
• Description: 1 A Synchronous Switching Regulator
• Manufacturer: Semtech Corporation

© 2000 SEMTECH CORP.
June 28, 2000
Typical Application Circuit
COMPONENT SELECTION
Input Capacitor
The Input capacitor(s) must be capable of 0.5A RMS
ripple current. Total capacitance values of 22uF to
47uF are suitable with Esr of less than 350m . Typical
capacitors would be AVX TPS Tantalum surface
mount type TPSC476M010R350 or Panasonic Spe-
cialty Polymer Aluminum Electrolytic surface mount
type ECGC0KB470RA. Other types of capacitor are
also suitable, such as high value ceramic or Aluminum
electrolytics.
Inductor
Suitable Inductor values are in the range of 47uH to
330uH, higher inductor values give lower output volt-
age ripple and slower transient response time. Inductor
ripple current acts with the output capacitor ESR to
produce output voltage ripple. For a given output volt-
age ripple requirement, L can be calculated from
Output Capacitor
The output capacitor requirements are similar to the
input capacitor except ripple current is lower. Similar
capacitors will be suitable.
L
5V in
V
IN
V
V
+
RIPPLE
OUT
C1
47uF
GND
ESR
f
V
IN
V
OUT
C3
0.033uF
1 A Synchronous Switching Regulator
1
2
3
4
SC1409IS
U1
SYNC/FSET
EN/SS
AGND
FB
PHASE
PGND
ISET
VIN
Soft Start Capacitor
The soft start capacitor value determines the soft start
time according to the following equation:-
C = 0.015t
where C is the soft start capacitor value in uF
and t is the soft start time in mS
Startup from a shutdown condition will take longer than
this since the soft start capacitor must charge from its
shutdown value to approximately 1V at a charge cur-
rent of 1uA before the shutdown is released and true
soft start begins.
Divider Resistors
The current through the divider chain should be at least
1000 times higher than the FB input bias current to
eliminate ratio errors. This gives a maximum value for
the bottom divider resistor (R4) of 24k
The top resistor (R2) can then be determined from
R2 = (Vo-1.203)·R4/1.203
Compensation
Normally, external compensation is not recommended,
however, if required, loop response can be tailored
with suitable values for R3, R5, C4 and C5. Consult the
factory for details.
OPEN
R1
652 MITCHELL ROAD NEWBURY PARK CA 91320
8
7
6
5
DO3316P-104
Coilcraft
100uH
L1
C6
0.1uF
R2
1.74k*
R4
1.0k
Place C6 across pins 7 and 5
of SC1409, as close as
possible to package
C4
OPEN
R3
OPEN
C5
OPEN
R5
OPEN
+
C2
47uF
3.3V Out
SC1409
7