A single-chip, all bipolar, camera control IC

Abstract

RECENT ADVANCES in silicon processing have made i t possible t o design a bipolar camera control IC, including the silicon photodiode, on a single chip; previous electronic camera control systems have used multiple chips or less desirable cadmium-sulphide, CdS, cells. This IC operates without external components and provides linear control of the shutter and two-position aperture selection in addition t o a low light warning and a battery check. Each die is calibrated during wafer sort using a reference light source by a computer controlled metal link blowing technique. The chip is mounted in an 8-pin DIL package, Figure 4, which is molded in a clear compound. A small recess is formed in the package t o allow a light filter to be convenientlyinserted, thereby reducing the infrared response. A second lay-er of aluminum is used on the die t o shield the low level control circuitry from the incident light. Current was used as the analog of scene brightness because of the dynamic range and compatibility with bipolar circuitry. Special processing was developed to provide a blue-enhanced photodiode and transistors with good hFE holdup to handle the low level currents. A shallow photodlode junction was created by use of ion implantation to boost the normalized conversion efficiency to 60% in the visible light range. Low temperature processing and anneals,provide typical NPN hFE s of 100 and PNP hFE s of 50 at 1OnA collector current, while simultaneouslyholding junction leakages t o less than 1pA at room temperature. The filtered scene illumination is linearly convertcd t o current in the photodiode. The input differential amplifier, Figure 2, biases this device and replicates the photocurrent in a multiplicity of outputs. For near zero leakage a bias of 0V is needed, but a requirement that the shutter time out in total darkness necessitated a circuit which encourages the diode t o provide some limited dark current. The diodes at the noninverting input arc fixed biased whereas those in series with the photodiode carry I), (-4nA/Fc). Should I x 0, the latter diodes have n o voltage drop and the photodiode would approach a 1 V reverse bias. This stimulates leakage currcnt which increases the diode drops and tends to minimize and thereby regulate the leakage. Photodiode current, whether thermal or light induced, is provided by the upper PNPs through the action of the loop. The Ib of the NPN at the inverting input represents an error source which is reduced to less than 1% by adaptively biasing the tail current of the differential amplifier. This light dependent biasing technique improves the transient response of the loop, which is necessary .to realize proper exposure time when light is returned from a flash-illuminated subject; earlier cameras, which used the slower CdS cell, could not respond to this rqturncd light and would therefore produce an overexposed picture. Sufficient loop gain is provided t o insure linearity over the incident light range of interest (“300:l) and the loop is frequency compensated by the junction capacitance, C;, of thc photodiode. Comparisons between photocurrent and various rcference currents take place at levels as low as 2nA. Therefore, it is necessary t o have a reference current generator that is free from the problem of compounding leakage and hFE errors in the current reduction Circuitry. Reinjector current division, Figure 3, minimizes these errors and can be fabricated in one epitaxial tub. The first PNP in the chain has a large valued, easily derived reference current (10pA) entered at the emitter. Collector scaling around this emitter provides an initial 10:1 current reduction. The smaller collector is tied to the emitter of the succccding device and both regions are left electrically uncommitted. TO maintain charge neutrality, this collected current must be reinjccted. Current reinjection is shared by the small collector and the next emitter based upon relative area. The next PNP structure behaves just like the common-base first device. A cascade of three provides a current division oi 5000:l and useful intermediate outputs can be obtained instead of shunting the surplus current to ground.