Circuit Note

CN-0537

Rev. A | Page 3 of 8

The EVAL-CN0537-ALGO algorithm was specifically designed

for detecting fire conditions using the ADPD188BI sensor

and optimized smoke chamber, as defined by the UL 217 and

UL 268 standards. The algorithm itself was tuned and verified

through a large data set, EVAL-CN0537-DATA, captured

from many ADPD188BI devices across all of the test scenarios

defined in Table 1. The tests were performed at certified

OSHA approved laboratories specializing in fire and life

safety. The EVAL-CN0537-DATA data set includes reference

measurements to understand sensor performance and alarm

conditions for different smoke sources. An example smoke

profile is shown in Figure 3, where the IR and blue sensor

measures are compared against the reference smoke obscuration

data under the Hamburger and Polyurethane test scenario.

Because battery powered devices are a common use case for

smoke detectors, the EVAL-CN0537-ALGO was designed

and built to minimize the output data required from the

ADPD188BI, along with the number of microcontroller

computations necessary while still meeting the strict UL 217

and UL 268 standards. For more details on the testing validation,

see the test results.

Figure 3. Smoke Profile for Burger Polyurethane Test

Calculating the Power Transfer Ratio

The smoke response of the ADPD188BI is best expressed as a

ratio of the received optical power to the transmitted optical

power. Referred to as the power transfer ratio (PTR), this

parameter is a much more meaningful value than the raw

output codes, as it is independent of the actual hardware

settings used. Additionally, obscuration level, the standard

unit of measurement for smoke detectors, is directly related

to the PTR, as shown in Equation 1.

PTR = γβ

(1)

where:

PTR is the power transfer ratio in nW/mW.

γ is the ADPD188BI scaling factor (0.64 typical for blue

light and 0.24 typical for infrared light).

β is the obscuration level in ft-1.

Figure 4. Blue LED PTR, IR LED PTR, and Obscuration Level

For each LED, the PTR can be calculated using Equation 2.

_

11

PD

PD

LEDx

LEDx PK

PD

LED

PI

PTR

PI

R

η

=

=

××

(2)

where:

PTR is the power transfer ratio of the LED in nW/mW.

IPD is the photodetector current in nA.

ILEDx_PK is the peak LED current in mA.

RPD is the photodetector responsivity in A/W (0.26 typical

for blue light and 0.41 typical for infrared light).

ηLEDx is the LED efficiency in W/A.

Calculating the ILED and IPD

The ADPD188BI allows for configuring the peak currents of

each LED through a combination of course and fine adjustment

registers and a current scale factor. The peak LED current can

be calculated using Equation 3 to Equation 6.

ILEDx_PK = ILEDx(COURSE) + ILEDx(FINE) + ILEDx(SCALE)

(3)

ILEDx(COURSE) = 50.3 + 19.8 × ILSEDx_COURSE

(4)

ILEDx(FINE) = 0.74 + 0.022 × ILEDx_FINE

(5)

ILEDx(SCALE) = 0.1 + 0.9 × ILEDx_SCALE

(6)

where:

ILEDx_PK is the peak LED current in mA.

ILEDx(COURSE) is the LED course current in mA.

ILEDx(FINE) is the LED fine current in mA.

ILEDx(SCALE) is the LED current scaling in mA.

ILEDx_COURSE, ILEDx_FINE, and ILEDx_SCALE are the

values for the control registers for each LED. For the integrated

blue LED, these can be set in Register Address 0x23 and

Register Address 0x25. For the integrated infrared LED,

these can be set in Register Address 0x22 and Register

Address 0x25. Note that in the ADPD188BI registers, the

blue LED is LEDX1 and the infrared LED is LEDX3.

8

–1

1

0

2

3

5

7

4

6

8

–1

1

0

2

3

5

7

4

6

0

1200

1400

1000

800

600

400

200

TIME (Seconds)

REFERENCE OBSCURATION

IR

BLUE

OBSCURATION

WINDOW

13.5

13.0

12.5

12.0

11.5

11.0

10.5

10.0

9.5

9.0

8.5

8.0

7.5

7.0

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

–0.5

6.5

4.4

4.2

4.0

3.8

3.6

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

1.8

–0.2

TIME (PM)

END

IGNITE

12%

ALARM THRESHOLD

AT 5% PER FOOT

OBSCURATION