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SC1408IS.TR
(Rev.:2000)

Low Voltage, Boost DC-DC Controller

Semtech Corporation 

LOW VOLTAGE, BOOST DC-DC

CONTROLLER

SC1408

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

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 VO +VIN and average current equal to the

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 100µF tanta-

lum will be adequate for most applications.

SEPIC Converter

Series Capacitors

RSENSE

The Series capacitor(s) must be capable of handling an

Again, with the SEPIC topology, the value of the sense RMS current given by:-

resistor is the primary determining factor for maximum

output current. The simplest approach to select RSENSE

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

IRMS = IO

VO + 0.5

VIN

Boost converter.

Feed Forward Capacitor

Output Voltage

The feed forward capacitor should be selected in the

Output voltage setting works exactly the same in SEPIC same way as for a Boost application.

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 LAYOUT GUIDELINES

its 16.5V rating. In the circuit of Fig.2: This requires max-

imum output voltage to be limited to 16.5V-Vin. Higher The SC1408 is fairly insensitive to layout, however fol-

output voltages are possible with different IC supply

lowing some simple guidelines will help ensure success-

strategies.

ful implementation.

Inductor

Whenever possible, lay the circuitry out over a ground

The SEPIC topology requires a coupled inductor. Again plane. Connect AGND and GND together at the IC and

a good choice for most applications is 22uH. Smaller in- return to the ground plane with one or two vias. Place

ductor values result in higher peak currents and increase the current sense resistor (R2), the REF decoupling ca-

output ripple, while larger values will result in slower loop pacitor (C5) and the divider resistors (R3 and R4) close

response.

to the IC, Return the ground side of these components

to the ground plane close to the AGND/GND pin vias.

Transistor Selection

Connect the top side of the sense resistor to ISENSE

The choice of FET can be critical, especially in battery with a short trace and the source of Q1 directly to the

powered applications where the converter must be able sense resistor. Make the output loop Q1, D1 and the

to use all of the available energy in the battery. This re- output capacitors (C3, C8) as small as possible to mini-

quires that the converter be capable of starting up from mize EMI.

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 VGS(th) is required. A good rule of

thumb is that VGS(th) should be at least 0.5V less than the

minimum input voltage.

© 2000 SEMTECH CORP.

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