The onsemi 4N28VM is a single-channel optoisolator in a 6-DIP through-hole package, with a DC input LED driving a phototransistor output that includes a base connection for external biasing or bandwidth control.
Switching performance and transfer ratio
Turn-on and turn-off times are 2 µs typical each — this gives a practical switching ceiling around 250 kHz for a 50 % duty cycle, though the base terminal can be used to speed up turn-off with a base-emitter resistor. The output transistor is rated for 30 V collector-emitter maximum, and the LED forward voltage is 1.18 V typical at rated current — the 60 mA absolute maximum forward current allows higher drive for applications that need more output current at the cost of CTR degradation over temperature.
No official successor or pin-compatible replacement is recorded from onsemi — any substitution requires a functional evaluation of the BOM position.