Mid range application design (LED supply from +8.5V)

The circuit of Figure 23 is a minimum part count design for a more powerful illumination (3 LEDs are equal to 50% more illumination ener -

gy).

The design considerations are exactly the same as before. The difference is the additional third LED3, which replaces the diodes D1A and

D1B, in order to have more powerful illumination. As it is used as a minimum part count design, the decoupling between the LED supply

circuit and the epc600 supply is not of the same quality as in the short range example before. Therefore, the layout design regarding the

noise and ripple voltage has to be done very carefully.

2.5. External LED driver

For a more powerful, sensitive or fast system application with an epc600 chip, the use of an external LED driver is an option. The design

has to take into consideration that the LEDs are modulated at high frequencies (e.g. 10MHz) and with high power. So far, the layout needs

to be well decoupled and with low Z conductors. The schematic has to follow the correct signal inversions as given in Figure 24. This is im-

portant in order to have the correct phase for the modulation.

Note: Take care that the voltage at pin LED do not exceed the specified operational range of maximum +5V.

epc600's pin LED is driven by an open drain switching transistor. The pull-up termination resistor R1 tied to the V_LOGIC supply (+5V)

guarantees safe voltage operating conditions for the output LED. The modulation signal of output LED feeds a fast inverting digital buffer

IC1. It drives the fast switching transistor T1. T1 switches the current through LED1-LEDn on/off in order to modulate the light. Output LED

= low corresponds to light = on.

As opposed to the on-chip driver, such a design can have additional light levels as the circuit in Figure 24 shows. An I/O port of the micro-

processor switches the intensity to the LOW or HIGH level. IC1 and IC2 are the necessary selecting logics to drive the transistors T1 and

T2. The illumination (current through LED1-LEDn) is controlled by R2 and R3 for the high and the low illumination level.

epc600

LED

GND

VDD

GND

C1

100nF

VDD +8.5V

Microprocessor

SCLK

SDATA

SCLK

SDATA

2-wire interface

HIGH

GNDLED

T1

LED1

LEDn

T2

R3

R2

+

C2

C3

GND_LED

V_LED

R1

V_LOGIC

LOW

IC1

IC2

Figure 24: Long range or fast response application (principal schematic)

Advantages of this circuit are: the free design of the illumination power (number of LEDs), the additional selectable light levels for extend -

ing the operational range, the independent voltage supply for the LEDs and the strong decoupling of the epc600 supply and LED circuitry.

© 2014 ESPROS Photonics Corporation

Characteristics subject to change without notice

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Datasheet epc600 - V1.3

www.espros.ch