ZXLD1374

ZXLD1374

Document number: DS35032 Rev. 1 - 2

Page 16 of 35

www.diodes.com

October 2010

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Diodes Incorporated

When the ADJ pin is directly connected to the REF pin, this becomes:

LED

2

GI

1

GI

1

GI

s

I

mV

225

)

R

(R

R

R

+

=

Note that the average LED current for a Boost or Buck-boost converter is always less than the average sense resistor

current. For the ZXLD1374, the recommended potential divider ratio is given by:

50

.

0

)

R

R

(

R

2

.

0

2

GI

1

GI

1

GI

≤

+

≤

It is possible to use a different combination of GI pin voltages and sense resistor values to set the LED current.

In general the design procedure to follow is:

-

-

Set output conditions in terms of LED current and the number of LEDs

-

Define controller topology – Buck, Boost or Buck-boost

Calculate the maximum duty-cycle as:

Buck mode

MIN

IN

LEDs

MAX

V

V

D

=

Boost mode

LEDS

MIN

IN

LEDS

MAX

V

V

V

D

−

=

Buck-boost mode

MIN

IN

LEDS

LEDS

MAX

V

V

V

D

+

=

MAX

2

GI

1

GI

1

GI

ADJ

GI

RATIO

D

1

)

R

(R

R

V

V

GI

−

≤

+

=

=

-

Set RGI1 as:

Ω

≤

≤

Ω

k

200

R

k

10

1

GI

-

Calculate RGI2 as:

1

GI

MAX

MAX

2

GI

R

x

D

1

D

R

−

≈

-

Calculate the sense resistor as:

LED

2

GI

1

GI

1

GI

s

I

mV

225

)

R

(R

R

R

+

=

If the potential divider ratio is greater than 0.64, the device detects that Buck-mode operation is desired and the output

current will deviate from the desired value.