7

 2005 Semtech Corp.

www.semtech.com

SC104

POWER MANAGEMENT

Step 3: (Cont.)

Using our 6 LED example:

L

thus I

Step 4: Calculating the Current Limit Resistor for

Discontinuous Mode

The current limit resistor value is calculated based upon

the minimum ratio of the switch current to the current out

of the LIM pin. It also takes into account the fact that

there is a propagation delay during which time the

inductor current ramps beyond the current limit trip point.

Since I

maximum. The maximum current limit resistor, R

discontinuous mode is therefore:

()

















−

•

−

•

=

)

D

(

)

SAT

(

CE

IN

plh

)

D

(

LIM

)

MIN

(

)

MIN

(

LIM

)

D

(

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

Using our 6 LED example:

I

thus R

Selecting this value or the next lower standard value in

this case gives us 7.5k

Ω. Of course a lower value resistor

may be used if desired, but may not necessarily be the

most efficient choice.

Step 5: Calculating the Current Set Resistor

The current set resistor is in series with the series LED

string. Thus the voltage developed across it is proportional

to the current flowing through the LEDs. The device will

regulate this voltage so that its average value equals the

feedback voltage, V

current set resistor value is given by the following

Applications Information (Cont.)

equation:

OUT

FB

SET

I

V

R

=

Using our 6 LED example:

I

thus R

We will select the 1% resistor value 23.2

Ω.

Note: this calculation is applicable to both continuous and

discontinuous modes.

Step 6: Calculating the Inductor for Continuous Mode

Having determined that we need to be operating in

continuous mode, we next need to calculate the

maximum inductor value allowed that will permit the part

to output the correct power. The maximum continuous

inductor value, L

()

OUT

OUT

IN

f

OUT

)

MIN

(

OFF

IN

)

C

(

I

V

4

.

1

V

V

V

t

V

2

L

•

•

−

+

•

•

•

=

Where:

I

t

Selecting the next lower standard value gives us a safe

value for this inductor.

Step 7: Calculating the Current Limit Required with this

Inductor for Continuous Mode

Having determined the inductor value we are going to

use, we next need to calculate the current limit required

to meet the necessary output power. The continuous

mode current limit, I

()

)

C

(

)

MIN

(

OFF

IN

f

OUT

)

C

(

LIM

L

4

.

0

t

V

V

V

I

•

•

−

+

=

Step 8: Calculating the Current Limit Resistor for

Continuous Mode

The current limit resistor value is calculated based upon

the minimum ratio of the switch current to the current

out of the LIM pin. It also takes into account the fact

that there is a propagation delay during which time the