2.4. On-chip LED driver

A feature for the minimum part count system is the on-chip LED driver. Figure 21, Figure 22 & Figure 23 show examples for possible cir-

cuits. The design has to take in consideration that these LEDs are switched very fast (e.g. 10MHz square-wave) and with high power

(<180mA). Suggested are high speed LEDs with short switching times e.g. Osram SFH4059, Vishay VSMB2000, Stanley DNK5306. The

layout needs to be well decoupled and with low Z conductors.

Notes:

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

■ The signal processing of the epc600 chip is based on a sine-wave modulated light-signal. All descriptions in this manual are related to

this. However, to simplify the LED driver circuit as well to reduce the power-losses of the LED, the chip uses a square-wave modulated

signal for the LED. This change together with all the other non-linear influences on the signal path leads to only a small error, which can

easily be corrected in the measurement data (refer to chapter 1.3.8. Calibration and compensations).

LEDs with a +5V supply

If a +5V supply is available, the LEDs can simply be supplied by this as shown in Figure 21. Resistor R1 limits the LED current <180mA.

For the LED power dissipation calculation, refer to chapter 3.3.6.: Cycle times & response time.

epc600

LED

GND

GND

LED1

SFH4059

LED2

SFH4059

R1

9R

C3

100nF

+

C4

10μF

Low Z, fast switching connection

GNDLED

VLED = 5V

GND_LED

Figure 21: Short range application

with +5Vdecoupled LED supply

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

The circuit in Figure 22 drives two power IR LEDs. The output LED is driven by an open drain switching transistor. The illumination intensi -

ty of the LEDs is defined by the current flowing through the resistor R1. For the output LED = on, the current of the resistor R1 is <180mA.

In order to have safe voltage operating conditions for the output LED (max. +5V), there is a minimum current of <1mA flowing through the

LED diodes and the D1 diodes in the off-state. This also gives the advantage of the enhanced switching performance for the LEDs. This

off-current is set by the resistor R2. C3 and C4 are the charge capacitors to supply the LEDs. In order to support the required fast switch -

ing, C3 shall be of a ceramic type. The decoupling of the supply of the epc600 chip is done by C1 and C2 (ceramic type). The resistor R3

decouples the LED feeding circuit from the ep600 supply. The voltage drop of the diodes D1A and D1B adjust the voltage supply level to

the LED supply level. The advantage of this concept is that it gives a good decoupling between the epc600- and the LED-supply.

For the LED power dissipation calculation, refer to chapter 3.3.6.: Cycle times & response time.

epc600

LED

GND

VDD

GND

VDD = 8.5V

LED1

SFH4059

LED2

SFH4059

R2

4k7

R1

27R

C3

100nF

+

C1

10μF

GND

C2

100nF

+

C4

10μF

R3

15R

D1A

BAV99

D1B

BAV99

Low Z, fast switching connection

GNDLED

epc600

LED

GND

VDD

GND

VDD = 8.5V

LED1

SFH4059

LED2

SFH4059

LED3

SFH4059

R3

4k7

R4

27R

C3

100nF

+

C1

10μF

GND

C2

100nF

Low Z, fast switching connection

GNDLED

Figure 22: Short range application

with a decoupled LED supply

Figure 23: Mid range application

with a minimum part count circuit

© 2014 ESPROS Photonics Corporation

Characteristics subject to change without notice

19 / 30

Datasheet epc600 - V1.3

www.espros.ch