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Application Note
AN3011
The Performance Comparison of 6-pin photocoupler with base connection
pin and Renesas 4-pin photocouplers used in digital mode
Authors: Van N. Tran CEL Staff Application Engineer, Opto Semiconductors
Larry Sisken CEL Product Marketing Manager, Opto Semiconductors
Wei Z. Jiang,Graduate Intern (MSEE), SJSU
Introduction
This paper intends to compare the performance of a 6-pin
photo-transistor coupler vs. Renesas 4-pin photo-tran-
sistor coupler in the digital mode. The CNY17-3, a 6-pin
photo-transistor coupler made by a few opto manufactur-
ers is used as a reference in the writing of this paper.
In general, the purpose of using a 6-pin photo-transistor
coupler with the base connection is to improve the
switching time of the device. The improvement is imple-
mented by biasing the base of the phototransistor with a
resistor, generally, in the range of 100K or greater con-
nected from its base to ground. However, if the switching
speed is not critical, many users would leave the base
connection pin of a photo-transistor coupler unconnected
or unused.
PCB Board Saving
In addition, some applications where the PCB board space
is a premium, the 6-pin optocoupler would take more space
compared to 4-pin optocoupler.
As a comparison of the PCB area taken by a 4-pin DIP vs.
6-pin Dip, the table below shows that the 4-pin DIP package
would save PCB board space by roughly a factor of 2.
Package
Package
Dimensions
(Max) in mm²
PS25xxL-1 (SMT)
4-pin DIP
47.72
CNY-17X (SMT)
6-pin DIP
91.57
Noise Issue
Unfortunately, in a noisy environment, the circuit may be-
come susceptible to noise when a 6-pin photo-transistor
coupler with the base connection is left unused. When the
base connection pin is not used, it would act as an antenna
and allow noise current to enter the base of the phototran-
sistor. As a result, the noise current would appear at its
output and its amplitude is multiplied by a factor of current
transfer ratio (CTR). The fix to the noise problem is to add
a LC circuit to the unused base connection pin to suppress
EMI noise, which means adding cost and components to
the BOM.
How Renesas Addresses The Problems
To address the above issues and different requirements of a
photocoupler with transistor output, Renesas has offered a
range of products in 4-pin optocouplers for a general purpose
application where the switching time is not crucial to a fast
switching device where the applications require higher data
rate.
For example, the PS2501A-1 is commonly used in the
application where the switching time is not crucial. However,
for faster switching time requirements, Renesas offers
PS2513-1 and the new PS2514-1 to meet the needs. The
new PS2514-1 incorporates an internal bias at the base of
the phototransistor. As a result, the new PS2514-1 has a
faster switching time without the need of adding an extra
external resistor to bias the base of the phototransistor and
it minimizes the EMI susceptibility as experienced with 6-pin
photocoupler with its base connection exposed. In addition,
the PS2514-1 offers PCB board saving as mentioned above.
The following pages shows the comparison table and figures
for the purpose of reference to show the performance of
Renesas photocouplers in reference to 6-pin photocoupler
on the switching time.
Page 1