SFH7773 datasheet(5/23 Pages) OSRAM

5 / 23 page

December 12, 2011

page 5 of 23

As a rule of thumb, 30 counts result in

almost a quadrupling in irradiance (PS

signal level) whereas 10 counts represent

roughly a factor of 1.55 in analog signal

level.

The

digital

proximity

count

signal

correlated to the detected irradiance Ee.

There

approximate

logarithmic

relationship between the digital PS signal

the analog signal level (irradiance):

370

017

burst

counts

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≈

⋅

−

⋅

Eq. (1)

The sensor’s design ensures that a touch of

human skin directly onto the sensor (no

airgap) delivers the maximum sensor count

(which depends on LED drive current and

integration time). This ensures that even in

this rare case a reliable operation is

ensured.

5.3 Radiation Characteristics of the PS-

LED

Fig. 10 presents the radiation characteristics

of the IR-LED. The characteristics might

influence the design of the cover glass

(aperture). The directional sensitivity of the

proximity sensor photodiode (detector) is

very similar to the emitter’s radiation

characteristics.

Please refer to Sec. 10.1 for a more detailed

discussion.

5.4 Crosstalk

In general, most proximity sensors are

hidden behind a cover glass. However, the

cover glass causes reflections which might

make it impossible for the sensor to

differentiate between a target reflection (e.g.

human skin) and the reflections from the

100

125

150

100

150

200

Proximity Count vs. Target Distance

Kodak White, 90 %, 100 x 100 mm

Target Distance / mm

300 us

750 us

1000 us

1500 us

Fig. 9: Proximity sensor signal count vs.

target distance and integration time (LED

drive current = 200 mA).

-90

-60

-30

100

± 30°

Angle / °

Short Axis

Long Axis

± 50°

LED

Radiation Characteristics of the IR-LED

Fig. 10: Radiation characteristics of the

proximity sensor LED.

100

125

150

100

150

200

Target Distance / mm

200 mA

100 mA

50 mA

Proximity Count vs. Target Distance

Kodak White, 90 %, 100 x 100 mm

Fig. 8: Proximity sensor signal count vs.

target distance and LED drive current

(integration time tburst = 750 us).

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