Figure 4.

recommended. The R1•CX2 time
constant provides overdrive for
both turn-on and turn-off of the
LED, shortening the optical trise
and tfall. In order to limit the
electrical ILED pulse trise and tfall
to 5-10 ns, TXD must be driven
with a pulse current Iih of approximately
ILED/125. See Figure 4.
The HSDL-1100 datasheet Electrical
Specifications show that at Vih
= 4.25 V, the required Iih 6.6 mA.
Using R1 = 560 ohms, Iih will be
sufficient for 5-10ns trise and tfall of
ILED, if Vih 4.25 V. If the chosen
I/O chip, controller chip, or EnDec
chip does not support Vih
4.25 V at the required Iih, then R1
and CX2 can be chosen to allow for
a lower Vih at the required Iih value.
R1 can be lowered from 560 ohms
to allow for Vih < 4.25 V. CX2
should then be increased to preserve
the peaking characteristic.
If the I/O chip or controller chip
cannot provide the required Iih
even at Voh = 2.4 V, then a buffer
chip can be used to drive TXD of
the HSDL-1100. The output of
the buffer chip should be able
to source current of at least
IOH = 6 mA in magnitude. LS/