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Low Power Current Regulator for Monolithic LED Displays

IP.com Disclosure Number: IPCOM000077137D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Blount, FT: AUTHOR

Abstract

A typical light-emitting diode (LED) driver chip and LED display chip interconnection is shown in Fig. 1. To light up any display, current is supplied from the drivers to the appropriate LED segment diodes (S1,...S7).

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Low Power Current Regulator for Monolithic LED Displays

A typical light-emitting diode (LED) driver chip and LED display chip interconnection is shown in Fig. 1. To light up any display, current is supplied from the drivers to the appropriate LED segment diodes (S1,...S7).

The greatest problem with LED displays is their power dissipation. To reduce the power, a low-voltage supply is generally used with a saturating transistor switch and a resistor to regulate the current. The low-voltage supply is limited by the voltage required across the resistor to regulate the current. Current regulation is very important to reduce the maximum power dissipation and control the LED brightness. Thus, the low-voltage supply must be large compared to collector-emitter voltage plus the LED voltage to minimize the effects of the LED voltage tolerance, + 150 mv, and the low-voltage supply tolerance, + 108.

The circuit, shown in Fig. 2, obtains the regulation from a high voltage, V2 and resistor RA, while most of the current flows from voltage supply V1. V1 can now be much lower than in prior circuits since the resistor has been eliminated. Thus, power has been drastically reduced. The circuit operation is as follows: the current through RA is IR = V2 - VD - V LED over RA VD is the base-emitter voltage. The same current flows in transistors T1 to TN, since all bases and emitters are in parallel. The LED current is thus I LED = (N+1) IR. The number of devices, N, is determined by the...