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REF192 Datasheet(PDF) 18 Page - Analog Devices |
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REF192 Datasheet(HTML) 18 Page - Analog Devices |
18 / 24 page ![]() REV. E –18– REF19x Series Membrane Switch Controlled Power Supply With output load currents in the tens of mA, the REF19x family of references can operate as a low dropout power supply in hand- held instrument applications. In the circuit shown in Figure 3, amembrane ON/OFF switch is used to control the operation of the reference. During an initial power-on condition, the SLEEP pin is held to GND by the 10 k Ω resistor. Recall that this condition disables (read: three-state) the REF19x output. When the mem- brane ON switch is pressed, the SLEEP pin is momentarily pulled to VIN, enabling the REF19x output. At this point, current through the 10 k Ω is reduced and the internal current source connected to the SLEEP pin takes control. Pin 3 assumes and remains at the same potential as VIN. When the membrane OFF switch is pressed, the SLEEP pin is momentarily connected to GND, which once again disables the REF19x output. NC NC VIN 1 2 3 4 8 7 6 5 NC NC OUTPUT ON OFF 10k 1k 5% REF19x 1 F TANT NC = NO CONNECT Figure 3. 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 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 4. Full time current limiting is used for protection of the pass transistor against shorts. U1 REF196 (SEE TABLE) R4 2 R1 1k R2 1.5k Q2 2N3906 C2 100 F/25V D1 1N4148 (SEE TEXT ON SLEEP) R3 1.82k C1 10 F/25V (TANTALUM) S F C3 0.1 F 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 Figure 4. A Boosted 3.3 V Reference with Current Limiting In this circuit, the power supply current of reference U1 flowing through R1–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 temperature related drift. Short circuit protection is provided by Q2, which clamps drive to Q1 at about 300 mA of load current with values as shown. With this separation of control and power functions, dc stability is optimum, allowing best advantage use of premium grade REF19x devices for U1. Of course, load management should still be exercised. A short, heavy, low DCR (dc resistance) conductor should be used from U1–6 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 dissi- pation below the package rating. This is accomplished by simply raising R4. A tantalum output capacitor is used at C1 for its low ESR (Equivalent Series Resistance), 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 shown as an option, and when used some cautions are in order. Because of the addi- tional active devices in the VS line to U1, direct drive to Pin 3 does not work as with an unbuffered REF19x device. To enable shutdown control, the connection to U1-2 is broken at the “X,” and diode D1 then allows a CMOS control source VC to drive U1-3 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. A Negative Precision Reference without Precision Resistors In many current-output CMOS DAC applications where the output signal voltage must be of the same polarity as the reference voltage, it is often required to reconfigure a current-switching DAC into a voltage-switching DAC through the use of 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 from the point that 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 through the use of an operational amplifier and a pair of matched resistors in an inverting configura- tion. The disadvantage to that approach is that the largest single source of error in the circuit is the relative matching of the resistors used. |
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