SC1408 Semtech Corporation, SC1408 Datasheet - Page 8

• Manufacturer Part Number: SC1408
• Description: LOW VOLTAGE/ BOOST DC-DC CONTROLLER
• Manufacturer: Semtech Corporation

August 22, 2000
Feed Forward Capacitor
Although converters based on the SC1408 are stable,
with no possibility of oscillation, they are susceptable to
“mode switching” at intermediate current levels. This
leads to increased ripple voltage and slightly reduced
efficiency. Mode switching can be eliminated by a suit-
able choice of C9. It is possible to empirically select C9.
Start with a value of about 100pF and adjust until all low
frequency output ripple is eliminated at the desired out-
put current.
See AN99-15 “SC1408 Operating Modes” for a full dis-
cussion of the origins of “Mode switching” and a more
analytical approach to the selection of C9.
COMPONENT SELECTION
SEPIC Converter
R
Again, with the SEPIC topology, the value of the sense
resistor is the primary determining factor for maximum
output current. The simplest approach to select R
is to add Vin to Vo and use this value as the output volt-
age in the output current curves or in the equation for
Boost converter.
Output Voltage
Output voltage setting works exactly the same in SEPIC
topology as in Boost, including the ability to set to 5V by
connecting the FB pin to GND. Care must be taken to
ensure that the IC supply (pin2; BST) does not exceed
its 16.5V rating. In the circuit of Fig.2: This requires max-
imum output voltage to be limited to 16.5V-Vin. Higher
output voltages are possible with different IC supply
strategies.
Inductor
The SEPIC topology requires a coupled
a good choice for most applications is 22uH. Smaller in-
ductor values result in higher peak currents and increase
output ripple, while larger values will result in slower loop
response.
Transistor Selection
The choice of FET can be critical, especially in battery
powered applications where the converter must be able
to use all of the available energy in the battery. This re-
quires that the converter be capable of starting up from
very low input voltages. For example a two cell alkaline
system’s terminal voltage will drop to 1.8V as it ap-
proaches full discharge. For these demanding applica-
tions, a FET with low V
thumb is that V
minimum input voltage.
© 2000 SEMTECH CORP.
SENSE
GS(th)
should be at least 0.5V less than the
GS(th)
is required. A good rule of
inductor.
LOW VOLTAGE, BOOST DC-DC
CONTROLLER
SENSE
Again
Diode
For most applications, a Schottky diode should be used
as the output rectifier. It will be subjected to reverse volt-
ages of at least V
output current. Industry standard 1N5817 series or an
equivalent surface mount part would be suitable.
Output Capacitors
Output capacitors should be low ESR to minimize ripple
voltage and maximize efficiency. Low ESR tantalums,
OSCONs or the newer Polymer capacitors should be
used.
Input Capacitors
Input capacitors on a SEPIC converter are less critical
than the output capacitors, since there are no fast cur-
rent pulses drawn from the input supply. A 100 F tanta-
lum will be adequate for most applications.
Series Capacitors
The Series capacitor(s) must be capable of handling an
RMS current given by:-
Feed Forward Capacitor
The feed forward capacitor should be selected in the
same way as for a Boost application.
LAYOUT GUIDELINES
The SC1408 is fairly insensitive to layout, however fol-
lowing some simple guidelines will help ensure success-
ful implementation.
Whenever possible, lay the circuitry out over a ground
plane. Connect AGND and GND together at the IC and
return to the ground plane with one or two vias. Place
the current sense resistor (R2), the REF decoupling ca-
pacitor (C5) and the divider resistors (R3 and R4) close
to the IC, Return the ground side of these components
to the ground plane close to the AGND/GND pin vias.
Connect the top side of the sense resistor to ISENSE
with a short trace and the source of Q1 directly to the
sense resistor. Make the output loop Q1, D1 and the
output capacitors (C3, C8) as small as possible to mini-
mize EMI.
I
RMS
652 MITCHELL ROAD NEWBURY PARK CA 91320
I
O
V
O
V
IN
0
5 .
O
+V
IN
and average current equal to the
SC1408
8