LDS8724【PDF 6/10 页】IC LED DRIVER PWM 25MA 8TDFN

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LDS8724

BLOCK DIAGRAM

Figure 2. LDS8724 Functional Block Diagram

BASIC OPERATION

At power-up, EN/PWM pin should be logic LOW. The

LDS8724 starts operating when EN/PWM pin is

asserted logic high. If the input voltage is sufficient to

regulate all LED currents, the device remains in

operating mode. The low dropout Current regulator

performs well at V OUT voltage up to 250 mV above

summary LED forward voltage significantly increasing

driver’s efficiency.

If the input voltage is insufficient or falls to a level

where the regulated currents cannot be maintained,

the Under-Voltage protection turns device off setting

it in shutdown mode.

The LDS8724 has soft start function that prevent high

input current spike at device’ wake-up.

The EN/PWM pin should be held low for more than

10 ms to completely turn device in low current

shutdown mode.

LED Current Setting

The maximum current sink value in the LEDS pin is

factory preset at 30 mA. An external resisror R FB

should be used to bypass current if more than one

string is connected. R FB = 0.25 V / (I x M – 0.03),

ohms, where 0.25 V is a feedback voltage, I – is

current value per string in ampers, and M is number

of strings connected in parallel.

Power dissipated at R FB is equal W = 0.0625/R FB ,

watt.

In case of three strings with 25 mA current per string

R FB = 0.25/(0.025 x 3 – 0.03) = 5.6 ? , and dissipated

power W = 0.0625/5.6 = 0.011 W.

The average LED string current that determines LED

brightness may be controlled applying PWM signal to

the EN/PWM pin. The maximum PWM frequency is

30 kHz, while frequiencies below 100 Hz are not

recommended to avoid visible LED flikering. Duty

cycle that determines average LED string current

may vary in the range from 5% to 100% at 1 kHz or

from 20% to 100% at 30 kHz with high linearity

current regulation.

Protection Mode

The output voltage V OUT is limited at about 36 V. This

is to prevent the output pin from exceeding its

absolute maximum rating if LED string is

disconnected or any LED.in string burns out creating

open circuitry.

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

enter a thermal protection shutdown mode. When the

device temperature drops by about 20°C, the device

will resume normal operation.

? 2010 IXYS Corp.

Characteristics subject to change without notice

6

Doc. No. 8724DS, Rev. N1.0

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