REF191ESZ datasheet(22/28 Pages) AD

22 / 28 page

REF19x Series

Rev. I | Page 22 of 28

ISY

ADJUST

RSET

IOUT

E.G., REF195: VOUT = 5V

IOUT = 5mA

R1 = 953Ω

P1 = 100Ω, 10-TURN

VIN

1µF

REF19x

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

Max

OUT

()

19x

REF

SET

OUT

()

19x

REF

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

VOUT (V)

5.0

3.3

4.5

5.0

VC*

U1/U2

REF195/

REF196

REF194/

REF195

*CMOS LOGIC LEVELS

+VS = 6V

VIN

COMMON

VOUT

COMMON

0.1µF

+VOUT

1µF

REF19x

(SEE TABLE)

REF19x

(SEE TABLE)

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).

REF191ESZ 数据表(PDF) 22 Page - Analog Devices

REF191ESZ 数据表(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表下载

链接网址

REF191ESZ 数据表(PDF) 22 Page - Analog Devices

REF191ESZ 数据表(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表 下载

链接网址

数据表下载