sccw77400a ON Semiconductor, sccw77400a Datasheet - Page 8

• Manufacturer Part Number: sccw77400a
• Description: Reflective Optical Switch.
• Manufacturer: ON Semiconductor

the appropriate switching threshold. The result of the
comparison is store in a latch and occurs at the output (see
Figure 10: Timing Diagram)
For shortening production test time the sleep time T
can be decreased by a “High” signal on pin TST.
The LED driver generates a regulated current pulse for the
LED anode for a defined period triggered by the sequencer
(see Figure 10: Timing Diagram). During the on-phase of
the LED current can be adapted by the R
connected to the pin PROG of the ASIC according to the
following formula:
The current gain G
of the external resistor R
programmed between typical 10mA and 58mA for R
superior to 450Ω.
For R
I
led
LED Driver
=
PROG
I
led
< 450 Ω refers to the electrical characteristics.
,
on
Figure 11. I
+
Figure 10: Timing Diagram
G
PROG
prog
.
has a typical value of 6. By means
R
V
LED,ON
prog
dd
PROG
ON Semiconductor Confidential Proprietary
vs. (1/R
the LED current can be
For R
PROG
PROG
)
PROG
> 450Ω.
resistor
http://onsemi.com
SCCW77400
SLEEP
PROG
8
The transimpedance amplifier (TIA) converts the collector
current from the phototransistor to a voltage. The TIA has a
low impedance input regulated voltage V
ratio metric to the supply voltage VDD.
The implemented Schmitt trigger uses correlated double
sampling (CDS). The first phototransistor readout is
performed with the LED turned off in order to detect the
ambient light level and the phototransistor leakage current.
This value is stored for further comparison. The second
measurement is done with LED on in order to detect the
reflective signal. The difference between the both samples
must be larger than the upper threshold I
the output of the Schmitt trigger into the “High” state.
Respectively, the difference between the both samples
must be lower than the lower threshold I
the “Low” state. If the difference is between both
thresholds, the Schmitt trigger does not change its state.
The hysteresis is performed by addition of the threshold
(I
to the ambient light current. The added threshold current is
I
formula) when output is “High”.
The bias current generator supplies the other blocks with
appropriate bias currents during the appropriate timing
phase.
ST,HIGH
THRESHOLD
TIA Amplifier
CDS Schmitt trigger
Bias
when output is “Low” and I
) current during the first phototransistor readout
Figure 12. Output vs. photocurrent
I
st
,
low
=
(
1
−
I
st
,
hyst
ST,LOW
)
×
I
st
ST,LOW
,
(see the following
high
ST,HIGH
CE,BIAS
to switch into
, witch is
to switch