LDS8720

© 2011 IXYS Corp.

7

Doc. No. 8720DS, Rev. N1.1

Characteristics subject to change without notice

If the input supply voltage falls below the under-

voltage threshold, switch is disabled, and the device

enters shutdown mode.

LED Selection

The number of the LEDs in string is limited by

maximum output voltage that cannot exceed over-

voltage protection level. We recommend using not

voltage is above 3.0 V and not more that 8 LEDs if

External Components Selection

The LDS8720 requires four external components

only. The recommended input capacitor value is

between 1.0 and 10uF, while the output capacitor

selection is function of desired output ripple, loop

1 µF.

The inductor should allow around 20% higher peak

table Electrical Operating Characteristics on page 2).

However,

the

maximum

ripple

current

through





























IN

d

F

LED

LIM

R

V

V

NV

I

I

I

2

, where

N – is number of LEDs per string

Vd - is a current regulator voltage drop = 0.2 V,

We recommend continuous conduction mode for

value as





IN

d

F

LED

R

V

V

NV

I

I







2

Inductor value L is a function of switching frequency,

input and output voltage and is determined by

following equation:



























IN

IN

PM

d

F

R

V

V

V

V

NV

f

I

L

1

1

1

, where

L - is an inductance, H, and

f - is a switching frequency, 700 kHz.

Inductor should have minimum DC resistance to

avoid driver’s efficiency degradation.

The equation for the output capacitor selection is:

f

V

V

NV

I

V

V

NV

C

R

d

F

OUT

IN

d

F

OUT

)

(

)

(









, where

For example:

1 µF is a good choice.

efficiency and driver’s stability.

Recommended Layout

In active mode, the driver switches internally at a high

frequency. We recommend minimize trace length to

all external capacitors and inductor. The input and

output ceramic capacitors (X5R or X7R type) should

located as close to the device’ pins as possible to

prevent from EMI distribution

A ground plane should cover the area under the

driver IC as well as the bypass capacitors. Short

be implemented with the use of multiple via. A copper

area matching the TDFN exposed pad (PAD) must

be connected to the ground plane underneath. The

use of multiple via improves the package heat

dissipation.

Figure 3. Recommended layout

.