LDS8681 datasheet(6/9 Pages) IXYS

6 / 9 page

LDS8681

Doc. No. 8681_DS, Rev. N1.0

Characteristics subject to change without notice

Unused LED Channels

For applications with only one LED, unused channel

can be connected to VOUT or both channels may be

teed together to provide sum of the currents set at

each channel.

Protection Modes

The LDS8681 has follow protection modes:

1. LED short to VOUT protection

If LED pin is shorted to VOUT, LED burned out

becomes as short circuit, or LED pin voltage is within

from

VOUT to (VOUT - 1.5V)

range,

LDS8681

recognizes this condition as “LED Short” and disables

this channel. If LED pin voltage is less than (Vout –

1.5V),

LDS8681

restores

LED

current

this

particular channel to programmed value.

2. VOUT Over-Voltage Protection

The charge pump’ output voltage VOUT automatically

limits at about 5.9 V maximum. This is to prevent the

output pin from exceeding its absolute maximum

rating.

3. VOUT Short Circuit Protection

If VOUT is shorted to ground before LDS8681 is

enabled, input current may increase up to 1 A within

20 µs after enable and is limited to ~ 90 mA after

that.

4. Over-Temperature Protection

If the die temperature exceeds +160°C, the driver will

enter

shutdown

mode. Device

restores

normal

operation after die temperature falls below 135°C.

5. Input Voltage Under-Voltage Lockout

If VIN falls below 2.3 V but above 1.7 V (typical value),

LDS8681 will restart when input voltage rises above

2.3 V called lockout mode. If VIN continues fall below

1.7 V, LDS8681 device will enter shutdown mode..

To restart device, set EN pin logic low for more than

50 ms and logic high after that..

6. Open LED Definition

When LED becomes an open circuit, device will enter

into 2-x mode unless this unused LED channel is

connected to VOUT directly.

LED Selection

If the power source is a Li-ion battery, LEDs with

lowest forward voltages are recommended to achieve

highest efficiency and extended operation on a single

battery charge.

LEDs with forward voltages (VF) ranging from 1.6 V to

4 V may be used. Charge pump operates in highest

efficiency when VF voltage is close to VIN voltage

multiplied by switching mode, i.e. VIN x 1 or VIN x 2

External Components

The driver requires three external ceramic capacitors

(C1, CIN, and COUT) X5R or X7R type. In 2-x charge

pump mode, an input bypass capacitor of 1µF is

sufficient.

In 1x mode, the device operates in linear mode and

does not introduce switching noise back onto the

supply.

Recommended Layout

In charge pump mode, the driver switches internally

at a high frequency. It is recommended to minimize

trace length to all capacitors. A ground plane should

cover the area under the driver IC as well as the

bypass capacitors. Short connection to ground on

capacitors CIN and COUT can 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 (see figure

below).

The ground plane on the top layer is connected to the

ground plane on the bottom layer through two vias to

imporove power dissipation.

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