In contemporary vehicular alternator system voltage regulators, it is commonplace to use temperature dependent voltage sources to synthesize battery charge profiles. These temperature dependent charge profiles are necessary, because these systems are used to charge batteries that can be most efficiently charged at different rates depending on ambient temperature proximate the battery. For instance, some of these systems rely on generation of reference signals that have substantially different temperature coefficients that are used to determine thresholds used in regulation and fault detection operations.
Prior art systems accomplish this requirement by independently synthesizing two temperature dependent reference signals, or voltages. These voltages are used independently or in combination to synthesize the appropriate charging and fault detection profiles, and associated thresholds, for the battery under charge. In FIG. 1 a typical prior art block diagram is illustrated. A temperature dependent voltage source 101 has an output 103 for providing a voltage having a temperature dependence of -680 ppm/.degree.K. (parts per million/degree Kelvin) which is equivalent to -10 mV/.degree.K., (millivolts/degree Kelvin) which is typical in a 12 volt battery charging system. Also, a second voltage source 105 provides an output 107 for generating a temperature independent voltage--here 0 ppm/.degree.K., or 0 mV/.degree.K. The outputs 103 and 107 are supplied to a charge regulator 109 which in turn controls an alternator 104 to generate a temperature dependent voltage to a battery 113. The temperature dependent voltage provides a charge profile dependent on the voltage provided at the output 103. Also the charge regulator 109 has a fault detection function dependent on the voltage provided at the output 107.
Typically, two bandgap voltage sources are used to construct these temperature predictable voltage sources. This is the cause of several problems. First, since there are two complete bandgap reference circuits, the size; complexity; and cost of the combined circuits is unnecessarily high. Secondarily, because these voltage sources 101 and 105 operate independently, the charge regulator 109 cannot expect the outputs 103 and 107 of the two voltage sources 101 and 105 to be accurately related over temperature. This is problematic because the charge regulator 109 requires that the outputs 103 and 107 of the two voltage sources 101 and 105 be closely related, in order to minimize the error of the desired charge profile and associated thresholds. If the outputs 103 and 107 of the two voltage sources 101 and 105 are not correlated, then the charge regulator 109 must be at least as inaccurate as the independence between the two voltage sources 101 and 105.
Furthermore, to improve any correlation between the two voltage sources 101 and 105, active trimming of critical components is required. This is process intensive and expensive to perform in a manufacturing environment. At a minimum, this dual active trimming process adds unnecessary circuit area and test time.
What is needed is an improved dual output temperature compensated voltage reference, that has a known voltage relationship over temperature between its dual outputs, is simpler, more accurate, more compact in size, lower in cost, and is more manufacturable.