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REF192ES-REEL7 Datasheet(PDF) 20 Page - Analog Devices |
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REF192ES-REEL7 Datasheet(HTML) 20 Page - Analog Devices |
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20 / 28 page  REF19x Series Rev. I | Page 20 of 28 ON OFF 10kΩ 1kΩ 5% NC NC VIN NC NC OUTPUT REF19x NC = NO CONNECT 8 7 6 5 1 2 3 4 1µF TANT + Figure 22. Membrane Switch-Controlled Power Supply CURRENT-BOOSTED REFERENCES WITH CURRENT LIMITING While the 30 mA rated output current of the REF19x series is higher than is typical of other reference ICs, it can be boosted to higher levels, if desired, with the addition of a simple external PNP transistor, as shown in Figure 23. Full-time current limiting is used to protect the pass transistor against shorts. U1 REF196 (SEE TABLE) R4 2Ω R1 1kΩ R2 1.5kΩ Q2 2N3906 C2 100µF 25V D1 R3 1.82kΩ C1 10µF/25V (TANTALUM) S F C3 0.1µF F S R1 Q1 TIP32A (SEE TEXT) +VS = 6V TO 9V (SEE TEXT) VS COMMON VC VOUT COMMON OUTPUT TABLE U1 REF192 REF193 REF196 REF194 REF195 VOUT (V) 2.5 3.0 3.3 4.5 5.0 +VOUT 3.3V @ 150mA 2 6 + 1N4148 (SEE TEXT ON SLEEP) 3 + 4 Figure 23. Boosted 3.3 V Referenced with Current Limiting In this circuit, the power supply current of reference U1 flowing through R1 to R2 develops a base drive for Q1, whose collector provides the bulk of the output current. With a typical gain of 100 in Q1 for 100 mA to 200 mA loads, U1 is never required to furnish more than a few mA, so this factor minimizes tempera- ture-related drift. Short-circuit protection is provided by Q2, which clamps the drive to Q1 at about 300 mA of load current, with values as shown in Figure 23. With this separation of control and power functions, dc stability is optimum, allowing most advantageous use of premium grade REF19x devices for U1. Of course, load management should still be exercised. A short, heavy, low dc resistance (DCR) conductor should be used from U1 to U6 to the VOUT Sense Point S, where the collector of Q1 connects to the load, Point F. Because of the current limiting configuration, the dropout voltage circuit is raised about 1.1 V over that of the REF19x devices, due to the VBE of Q1 and the drop across Current Sense Resistor R4. However, overall dropout is typically still low enough to allow operation of a 5 V to 3.3 V regulator/reference using the REF196 for U1 as noted, with a VS as low as 4.5 V and a load current of 150 mA. The requirement for a heat sink on Q1 depends on the maximum input voltage and short-circuit current. With VS = 5 V and a 300 mA current limit, the worst-case dissipation of Q1 is 1.5 W, less than the TO-220 package 2 W limit. However, if smaller TO-39 or TO-5 packaged devices, such as the 2N4033, are used, the current limit should be reduced to keep maximum dissipation below the package rating. This is accomplished by simply raising R4. A tantalum output capacitor is used at C1 for its low equivalent series resistance (ESR), and the higher value is required for stability. Capacitor C2 provides input bypassing and can be an ordinary electrolytic. Shutdown control of the booster stage is an option, and when used, some cautions are needed. Due to the additional active devices in the VS line to U1, a direct drive to Pin 3 does not work as with an unbuffered REF19x device. To enable shutdown control, the connection from U1 to U2 is broken at the X, and Diode D1 then allows a CMOS control source, VC, to drive U1 to U3 for on/off operation. Startup from shutdown is not as clean under heavy load as it is in basic REF19x series, and can require several milliseconds under load. Nevertheless, it is still effective and can fully control 150 mA loads. When shutdown control is used, heavy capacitive loads should be minimized. NEGATIVE PRECISION REFERENCE WITHOUT PRECISION RESISTORS In many current-output CMOS DAC applications where the output signal voltage must be the same polarity as the reference voltage, it is often necessary to reconfigure a current-switching DAC into a voltage-switching DAC using a 1.25 V reference, an op amp, and a pair of resistors. Using a current-switching DAC directly requires an additional operational amplifier at the output to reinvert the signal. A negative voltage reference is then desirable, because an additional operational amplifier is not required for either reinversion (current-switching mode) or amplification (voltage-switching mode) of the DAC output voltage. In general, any positive voltage reference can be converted into a negative voltage reference using an operational amplifier and a pair of matched resistors in an inverting configuration. The disadvantage to this approach is that the largest single source of error in the circuit is the relative matching of the resistors used. The circuit illustrated in Figure 24 avoids the need for tightly matched resistors by using an active integrator circuit. In this circuit, the output of the voltage reference provides the input drive for the integrator. To maintain circuit equilibrium, the integrator adjusts its output to establish the proper relationship between the reference’s VOUT and GND. Thus, any desired negative output voltage can be selected by substituting for the appropriate reference IC. The sleep feature is maintained in the circuit with the simple addition of a PNP transistor and a 10 kΩ resistor. |
Similar Part No. - REF192ES-REEL7 |
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Similar Description - REF192ES-REEL7 |
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