© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

LOW VOLTAGE, BOOST DC-DC

CONTROLLER

SC1408

August 22, 2000

7

THEORY OF OPERATION

The SC1408 is a modified hysteretic boost converter

controller. The power switch is turned on when the out-

put voltage falls slightly below it’s setpoint. It remains on

for approximately 16

µs, or until the inductor current

reaches limit, whichever occurs first. The power switch is

then turned off for 2.3

µs, or until the output voltage once

again falls below setpoint, whichever occurs last.

The SC1408 is normally powered from the output volt-

age. Internal circuitry, such as the bandgap, compara-

tors and one shots will not function properly until the BST

pin voltage reaches 2.5V. To ensure startup at low input

voltages, the normal control circuitry is disabled and a

special, low voltage start up oscillator generates an ap-

proximate square wave at the GATE pin, initiating boost

action. When the output voltage reaches 2.5V, the nor-

mal control circuitry is enabled and the start up oscillator

shuts down.

To conserve power, a SHDN pin is provided which,

when pulled high, shuts down most internal circuitry. The

output voltage will then be one diode drop below the in-

put.

COMPONENT SELECTION

Boost Converter

The value of the sense resistor is the primary determin-

ing factor for maximum output current. The SC1408 has

a fixed current limit voltage threshold, which is devel-

oped by the peak inductor current flowing through

mum output current curves or from the equation below:

In the equation above, the use of 2.3µs for t

to slightly optimistic current values for low V

The theoretical curves use the actual value of t

V

for L=22

µH.

Output Voltage

Output voltage can be set to 5V by connecting the FB

pin to GND, or to any voltage in the 3.0V to 16.5V range

using external divider resistors.

The bottom resistor in the divider chain (R4) should be

300k

Ω or less and the top resistor (R3 in the application

circuits) can be calculated from:

Inductor

The SC1408 will work with a wide range of inductor val-

ues. A good choice for most applications is 22

µH.

Smaller inductor values result in higher peak currents

and increased output ripple, while larger values will re-

sult in slower loop response.

Transistor Selection

Normally the power switch will be an N-channel MOS-

FET, although in certain circumstances an NPN bipolar

may be substituted.

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.

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

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. Ripple voltage will be approximately:

Input Capacitors

Input capacitors on a boost converter are less critical

than the output capacitors, since there are no fast cur-

rent pulses drawn from the input supply. A 100

µF tanta-

lum will be adequate for most applications.

()(

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