8

 2005 Semtech Corp.

www.semtech.com

SC104

POWER MANAGEMENT

Applications Information (Cont.)

Step 8: (Cont.)

inductor current ramps beyond the current limit trip point.

Since I

maximum. The maximum current limit resistor, R

continuous mode is therefore:

()

















−

•

−

•

=

)

C

(

)

SAT

(

CE

IN

plh

)

C

(

LIM

)

MIN

(

)

MIN

(

LIM

)

C

(

LIM

L

V

V

t

I

Ratio

V

R

Where:

V

reference = 385mV

Ratio

= 7100 A/A

t

trip point to the power switch turning off = 200ns

Selecting the calculated value or the next lower

standard value is recommended.

Component Selection - General

Capacitor Selection: the SC104 has been designed to

be used with ceramic input and output capacitors. The

input to the device should be bypassed using a 4.7µF

ceramic capacitor rated for the maximum input voltage.

The output capacitor should be a ceramic capacitor in

the range of 0.22µF to 1µF. Care should be taken that

the voltage rating of this capacitor meets the output

voltage requriements, and if the part is going to be run

open circuit during assembly testing using the OVP

feature, then the capacitor should be rated 50V.

Schottky diode: any schottky diode rated for the

average and peak currents being seen in the circuit will

suffice. However a higher current rated schottky diode

will result in lower forward voltage drops and hence higher

efficiency. The selection of the schottky will depend upon

the optimum choice between efficiency, board space, and

cost.

Inductor: similarly, any inductor rated for the average and

peak currents required by the design and capable of

operating at the fixed off-time of 760ns will suffice, but

inductors with lower series resistance will result in lower

losses. The selection of the inductor will depend upon

the optimum choice between efficiency, board space, com-

ponent height, and cost. Toko D62LCB and D63LCB se-

ries work very well.

LED Dimming

Dimming the LEDs (i.e. reducing the output current from

the set level to reduce brightness) can be achieved a

couple of ways:

1) PWMing the ADJ pin using an open drain or open

collector (with no pull-up). Using a PWM signal at this pin

will reduce the output current by alternating between OFF

(ADJ < 100mV) and switching normally. The lower the

duty cycle, the lower the output current. A PWM frequency

of 1kHz maximum is recommended.

2) analog voltage applied to the ADJ pin: apply a DC

voltage between 0V to 0.1V (OFF) and 1V (full current) to

the ADJ pin. This pin should not be pulled above 1V

under normal operation, and should never exceed the

absolute maximum rating. Using a DC voltage will result

in the modulation frequency of the inductor current ramp

reducing and care must be taken to ensure that this does

not become audible in sensitive applications.