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REF192ES-REEL7 Datasheet(PDF) 22 Page - Analog Devices |
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REF192ES-REEL7 Datasheet(HTML) 22 Page - Analog Devices |
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22 / 28 page  REF19x Series Rev. I | Page 22 of 28 ISY ADJUST R1 RSET P1 RL IOUT E.G., REF195: VOUT = 5V IOUT = 5mA R1 = 953Ω P1 = 100Ω, 10-TURN VIN 1µF REF19x 2 3 4 6 VIN ≥ IOUT × RL (MAX) + VSY (MIN) IOUT = VOUT + ISY (REF19x) RSET VOUT >> ISY RSET VIN GND VREF SLEEP Figure 27. A Low Dropout, Precision Current Source The governing equations for the circuit are () ( ) 19x REF Min V Max R I V SY L OUT IN , + × = () 19x REF I R V I SY SET OUT OUT + = () 19x REF I R V SY SET OUT 〉〉 SWITCHED OUTPUT 5 V/3.3 V REFERENCE Applications often require digital control of reference voltages, selecting between one stable voltage and a second. With the sleep feature inherent to the REF19x series, switched output reference configurations are easily implemented with little additional hardware. The circuit in Figure 28 illustrates the general technique, which takes advantage of the output wire-OR capability of the REF19x device family. When off, a REF19x device is effectively an open circuit at the output node with respect to the power supply. When on, a REF19x device can source current up to its current rating, but sink only a few μA (essentially, just the relatively low current of the internal output scaling divider). Consequently, when two devices are wired together at their common outputs, the output voltage is the same as the output voltage for the on device. The off state device draws a small standby current of 15 μA (max), but otherwise does not interfere with operation of the on device, which can operate to its full current rating. Note that the two devices in the circuit conveniently share both input and output capacitors, and with CMOS logic drive, it is power efficient. U3B 74HC04 U3A 74HC04 VC VOUT (V) 5.0 3.3 4.5 5.0 VC* HI LO HI LO U1/U2 REF195/ REF196 REF194/ REF195 *CMOS LOGIC LEVELS +VS = 6V VIN COMMON VOUT COMMON C1 0.1µF +VOUT C2 1µF U1 REF19x (SEE TABLE) 2 3 4 U2 REF19x (SEE TABLE) 2 3 4 + 6 6 13 24 OUTPUT TABLE Figure 28. Switched Output Reference Using dissimilar REF19x series devices with this configuration allows logic selection between the U1/U2-specified terminal voltages. For example, with U1 (a REF195) and U2 (a REF196), as noted in the table in Figure 28, changing the CMOS- compatible VC logic control voltage from HI to LO selects between a nominal output of 5 V and 3.3 V, and vice versa. Other REF19x family units can also be used for U1/U2, with similar operation in a logic sense, but with outputs as per the individual paired devices (see the table in Figure 28). Of course, the exact output voltage tolerance, drift, and overall quality of the reference voltage is consistent with the grade of individual U1 and U2 devices. Due to the nature of the wire-OR, one application caveat should be understood about this circuit. Since U1 and U2 can only source current effectively, negative going output voltage changes, which require the sinking of current, necessarily takes longer than positive going changes. In practice, this means that the circuit is quite fast when undergoing a transition from 3.3 V to 5 V, but the transition from 5 V to 3.3 V takes longer. Exactly how much longer is a function of the load resistance, RL, seen at the output and the typical 1 μF value of C2. In general, a conservative transition time is approximately several milliseconds for load resistances in the range of 100 Ω to 1 kΩ. Note that for highest accuracy at the new output voltage, several time constants should be allowed (>7.6 time constants for <1/2 LSB error @ 10 bits, for example). KELVIN CONNECTIONS In many portable applications where the PC board cost and area go hand-in-hand, circuit interconnects are very often narrow. These narrow lines can cause large voltage drops if the voltage reference is required to provide load currents to various functions. The interconnections of a circuit can exhibit a typical line resistance of 0.45 mΩ/square (1 oz. Cu, for example). |
Similar Part No. - REF192ES-REEL7 |
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Similar Description - REF192ES-REEL7 |
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