Browse Prior Art Database

High Performance LED Driver

IP.com Disclosure Number: IPCOM000053044D
Original Publication Date: 1981-Aug-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 3 page(s) / 72K

Publishing Venue

IBM

Related People

DaCosta, RE: AUTHOR [+4]

Abstract

Existing fiber-optic driver circuits have a delay skew due to the switching of large currents and asymmetrical transient response of light-emitting diodes (LEDs). This circuit is designed to provide the fast collapse of current in a LED without benefit of an off-chip capacitor.

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High Performance LED Driver

Existing fiber-optic driver circuits have a delay skew due to the switching of large currents and asymmetrical transient response of light-emitting diodes (LEDs). This circuit is designed to provide the fast collapse of current in a LED without benefit of an off-chip capacitor.

The problem with present LED fiber-optic links is the relatively slow collapse of the LED current, as depicted in Fig. 1. This results in low bandpass. The solution (Fig. 2) is to force the voltage across the LED to zero, so that LED current declines faster. This has been accomplished in the past with off-chip capacitances.

Fig. 3 shows the high performance drive circuit providing the fast pull-up. The on current (Ion) of the LED is 150 ma and the standby current is 15 ma. In this driver circuit QAN and QAO are used to capacitively couple a pulse to the base of QAR. The positive-going pulse is coupled through the coupling transistors" base-collector diffusion capacitance. The value of this capacitance is a function of the original forward current through the base (P) and collector (N) junction of QAN and QAO.

The peak magnitude of the transient pulse is a function of the collector current of QAT which sets the forward current of the capacitive coupling diodes. (The most positive value that the emitter of QAR can attain is one Vbe above Vcc.) Transistor QAS controls the pulse width and the duration of the reverse bias of the LED. The current waveform of the LED i...