LDS8621

© 2009 IXYS Corp.

5

Doc. No. 8621DS, Rev. N1.0

Characteristics subject to change without notice

parallel to provide maximum current through one LED

in flash mode, the lower average current value in

torch mode may be achieved using PWM. PWM

signal may be applied to one of the PWM pins, while

other channel is off, or to both PWM pins. The

average LED current is proportional PWM duty cycle

and number of channels that are in ON state. For

instance, if both channels are connected in parallel to

provide 196 mA through one LED in flash mode,

turning one channel of in torch mode redjuces current

to 96 mA, while applying PWM signal with 50% duty

cycle to this channel reduces average LED current to

48 mA. LDS8621 allows modulation frequiencies in

the range from 100 Hz to 100000 kHz with duty cycle

from 100% to 1%. Lower than 100 Hz modulation

frequiencies is not recommended especialy at short

duty cycles because LED flicker may be visible.

The LDS8621 has follow protection modes:

becomes as short circuit, or LED pin voltage is within

condition as “LED Short” and disables this channel

with 750 µA control current. If LED pin voltage is less

than (Vout – 1.5V), LDS8621 restores LED current at

this particular channel to programmed value.

limits at about 6.2 V maximum. This is to prevent the

output pin from exceeding its absolute maximum

rating.

enabled, input current may increase up to 200 – 300

mA within 20 µs after enable and is limited to 35 – 40

mA after that.

4. Over-Temperature Protection

If the die temperature exceeds +150°C, the driver will

enter shutdown mode. The LDS8621 requires restart

after die temperature falls below 130°C.

5. Input Voltage Under-Voltage Lockout

enters shutdown mode. Device requires restart when

input voltage rises above 2.3 V.

open circuit, or if any LED at active channels is

disconnected,

LDS8621

starts

subsequently

changing modes (2x – 1x – 1.33x -1.5x – 2x -…) in

an attempt to compensate insufficient voltage. As a

result, average current at all other channels that are

ON may be below regulated level.

LED Selection

4.5V may be used. Charge pumps operate in the

Vin voltage multiplied by switching mode, i.e. Vin x 1,

Vin x 1.33 and so on. If the voltage source is a Li-ion

3.6 V to extend the battery life and achieve highest

efficiency.

External Components

The LDS8621 requires two external 1µF ceramic

capacitors (X5R or X7R type) for decoupling input,

output, and two 0.22 µF capacitors the same type for

the charge pump. In all charge pump modes, the

input current ripple is very low, and 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 four 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

TQFN exposed pad (PAD) must be connected to the

ground plane underneath. The use of multiple via

improves the package heat dissipation.