The REF20xx are a family of dual-output, VREF and VBIAS (VREF / 2) band-gap voltage references. The Functional Block Diagram section provides a block diagram of the basic band-gap topology and the two buffers used to derive the VREF and VBIAS outputs. Transistors Q1 and Q2 are biased such that the current density of Q1 is greater than that of Q2. The difference of the two base emitter voltages (VBE1 – VBE2) has a positive temperature coefficient and is forced across resistor R5. The voltage is amplified and added to the base emitter voltage of Q2, which has a negative temperature coefficient. The resulting band-gap output voltage is almost independent of temperature. Two independent buffers are used to generate VREF and VBIAS from the band-gap voltage. The resistors R1, R2 and R3, R4 are sized such that VBIAS = VREF / 2.
e-Trim™ is a method of package-level trim for the initial accuracy and temperature coefficient of VREF and VBIAS, implemented during the final steps of manufacturing after the plastic molding process. This method minimizes the influence of inherent transistor mismatch, as well as errors induced during package molding. e-Trim is implemented in the REF20xx to minimize the temperature drift and maximize the initial accuracy of both the VREF and VBIAS outputs.