 2001 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA

LH1525AT/AAB/AABTR

www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)

3–113

March 21, 2001-14

Applications

Input Control

The LH1525 low turn-on current SSR has highly sensitive photo-

detection circuits that will detect even the most minute currents

flowing through the LED. Leakage current must be considered

when designing a circuit to turn on and off these relays.

Figure 19 shows a typical logic circuit for providing LED drive

current. R1 is the input resistor that limits the amount of current

flowing through the LED. For 5.0 V operation, a 2700

Ω resistor

will limit the drive current to about 1.4 mA. Where high-speed

actuation is desirable, use a lower value resistor for R1. An

additional RC peaking circuit is not required with the LH1525

relay.

R2 is an optional pull-up resistor which pulls the logic level high

is set at a high value to minimize the overall current drawn from

the VS. The primary purpose of this resistor is to keep the differ-

ential voltage across the LED below its turn-on threshold. LED

dropout voltage is graphed vs. temperature in the Typical Per-

formance Characteristics section. When the logic gate is high,

leakage current will flow through R2. R2 will draw up to 8 mA

before developing a voltage potential which might possibly turn

on the LED.

Many applications will operate satisfactorily without a pull-up

resistor. In the logic circuit in Figure 1 the only path for current to

flow is back into the logic gate. Logic leakage is usually negligi-

ble. Each application should be evaluated, however, over the

full operating temperature range to make sure that leakage cur-

rent through the input control LED is kept to a value less than

the minimum LED forward current for switch turn-off

specification.

Figure 22. Input Control Circuit

ANY TTL OR

BUFFERED CMOS

LOGIC

2700

Ω

100 k

Ω

SSR