5

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1408

Applications Information

Theory of Operation

The SC1408 is a modified hysteretic boost converter

controller. The power switch is turned on when the output

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

approximately 16µs, 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 voltage.

Internal circuitry, such as the bandgap, comparators and

one shots, will not function properly until the BST pin volt-

age reaches 2.5V. To ensure start-up at low input volt-

ages, the normal control circuitry is disabled and a spe-

cial, low voltage start up oscillator generates an approxi-

mate square wave at the GATE pin, initiating boost action.

When the output voltage reaches 2.5V, the normal 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 out-

put voltage will then be 1 diode drop below the input.

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-

put current curves or from the equation below:

()(

)

DCR

Inductor

and

R

FET,

across

Voltage

V

Drop

Voltage

Forward

Diode

Output

V

:

Where

V

V

V

V

V

V

V

V

L

2

t

V

V

V

V

V

V

1

R

V

I

SENSE

FET

F

FET

F

O

IN

F

O

FET

IN

off

FET

F

O

IN

F

O

SENSE

CS

)

MAX

(

O

=

=









−

+

−

+

−

−









−

+

−

+

−

=

In the equation above, the use of 2.3

∝s for t

slightly optimistic current values for low V

theoretical curves use the actual value of t

V

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 in the typical

application circuits) should be 300k

Ω or less and the top

resistor (R3 in the application circuits) can be calculated

from









−

=

1

V

V

4

R

3

R

REF

O

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 in-

crease output ripple, while larger values will result in

slower loop response.

Transistor selection

Normally the power switch will be an N-channel MOSFET,

although in certain circumstances an NPN bipolar may

be substituted.

The choice of FET can be critical, especially in battery pow-

ered applications where the converter must be able to use

all of the available energy in the battery. This requires that

the converter be capable of starting up from very low in-

put voltages. For example a two cell alkaline system’s ter-

minal voltage will drop to 1.8V as it approaches full dis-

charge. For these demanding applications, a FET with low

V

be at least 0.5V less than the 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

what less than the Inductor peak current. Industry stan-

dard 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 tantalum or

OSCON 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 current pulses

drawn from the input supply. A 100

µF tantalum will be

adequate for most applications.