CPC1590

R01

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7

4. Device Configuration

4.1 LED resistor

The input resistor is required to limit LED current to a

value set by Recommended Operating Conditions in

“Recommended Operating Conditions” on page 3. In some

cases, higher LED operating current would improve

driver speed; however, this higher current could also

reduce LED lifespan, which would cause reliability

issues.

The general equation used to calculate the resistor

value is:

collector-emitter voltage of the driving logic transis-

tor. (This parameter is provided in the manufacturer’s

data sheet.)

When calculating the resistor value, the designer

should take into consideration power-supply

variations, which can range about ±10%, temperature

variations from -40°C to +85°C, LED forward voltage

drop over the temperature range, and the resistor’s

tolerance and temperature stability rating.

When the LED resistor value is selected by the above

obtained from the following equation:

With power dissipation calculated, it is now possible to

select an appropriate resistor size that can be used in

the particular application circuit. It is recommended

that a resistor with at least twice the calculated power

rating should be selected.

4.2 Storage Capacitor

turn on a power MOSFET faster by delivering a

reservoir of charge to the gate. Selection of the

storage capacitor is given by the following equation:

This equation shows that the storage capacitor needs

to deliver enough charge to the gate while only

dropping 0.5V. The CPC1590 can deliver 32nC of

charge at rated operating speed, and will operate with

much larger loads, >4nF, with slower turn-on and

turn-off times.

The CPC1590 has an internal J-FET, which is used to

regulate the voltage applied to the storage capacitor.

The voltage applied to the storage capacitor will be

2

3

1

4

NC

NC

8

5

6

7

CPC1590

CAP

+V

LOAD

G

-V

LOAD

C

ST

A

B

R

LED

V

IN

LOAD

LED