ADR391(RevC) データシートの表示（PDF） - Analog Devices

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ADR391
(Rev.:RevC)

Precision Low Drift 2.048 V/2.5 V/4.096 V/5.0 V SOT-23 Reference with Shutdown

Analog Devices 

ADR390/ADR391/ADR392/ADR395

OUTPUT TABLE

U1/U2

VOUT1 (V) VOUT2 (V)

ADR390/ADR390

ADR391/ADR391

ADR392/ADR392

ADR395/ADR395

2.048

2.5

4.096

5

4.096

5.0

8.192

10

VIN

2 U2

VIN

C2

0.1␮F

1

SHDN

4

VOUT(F)

3

VOUT(S)

GND

5

VOUT2

C2

0.1␮F

2 U1

VIN

1

SHDN

4

VOUT(F)

3

VOUT(S)

GND

5

VOUT1

Figure 4. Stacking Voltage References with the

ADR390/ADR391/ADR392/ADR395

Two reference ICs are used, fed from an unregulated input, VIN.

The outputs of the individual ICs are simply connected in series,

which provides two output voltages VOUT1 and VOUT2. VOUT1 is

the terminal voltage of U1, while VOUT2 is the sum of this voltage

and the terminal voltage of U2. U1 and U2 are simply chosen for

the two voltages that supply the required outputs (see Output

Table). For example, if both U1 and U2 are ADR391s, VOUT1

is 2.5 V and VOUT2 is 5.0 V.

While this concept is simple, a precaution is in order. Since the

lower reference circuit must sink a small bias current from U2,

plus the base current from the series PNP output transistor in

U2, either the external load of U1 or R1 must provide a path for

this current. If the U1 minimum load is not well defined, the

resistor R1 should be used, set to a value that will conservatively

pass 600 µA of current with the applicable VOUT1 across it. Note

that the two U1 and U2 reference circuits are locally treated as

macrocells, each having its own bypasses at input and output for

best stability. Both U1 and U2 in this circuit can source dc

currents up to their full rating. The minimum input voltage, VIN,

is determined by the sum of the outputs, VOUT2, plus the drop-

out voltage of U2.

A Negative Precision Reference without Precision Resistors

A negative reference can be easily generated by adding an op amp,

A1, and configured as shown in Figure 5. VOUTF and VOUTS are

at virtual ground and therefore the negative reference can be

taken directly from the output of the op amp. The op amp must

be dual supply, low offset, and rail-to-rail if the negative supply

voltage is close to the reference output.

+VDD

2

VIN

4 VOUT(F)

3 VOUT(S) SHDN 1

GND

5

A1

–VREF

–VDD

Figure 5. Negative Reference

General-Purpose Current Source

Many times in low power applications, the need arises for a preci-

sion current source that can operate on low supply voltages. As

shown in Figure 6, the ADR390/ADR391/ADR392/ADR395

can be configured as a precision current source. The circuit

configuration illustrated is a floating current source with a

grounded load. The reference’s output voltage is bootstrapped

across RSET, which sets the output current into the load. With

this configuration, circuit precision is maintained for load cur-

rents in the range from the reference’s supply current, typically

90 µA to approximately 5 mA.

VIN

SHDN

VOUT

ADR39x

VIN VOUT

GND

ISY (ISET)

ISET

R1

0.1␮F

ISY

ADJUST

}R1

RSET

P1

IOUT = ISET + ISY (ISET)

RL

Figure 6. A General-Purpose Current Source

High Power Performance with Current Limit

In some cases, the user may want higher output current delivered

to a load and still achieve better than 0.5% accuracy out of the

ADR39x. The accuracy for a reference is normally specified on

the data sheet with no load. However, the output voltage changes

with load current.

The circuit in Figure 7 provides high current without compro-

mising the accuracy of the ADR39x. The series pass transistor

Q1 provides up to 1 A load current. The ADR39x delivers only

the base drive to Q1 through the force pin. The sense pin of the

ADR39x is a regulated output and is connected to the load.

The transistor Q2 protects Q1 during short circuit limit faults by

robbing its base drive. The maximum current is ILMAX ≈ 0.6 V/RS.

REV. C

–13–

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