REF198GBC View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

REF198GBC

0.3-18V; precision micropower, low dropout, voltage reference. For portable instrumentation, A-to-D and D-to-A converters, smart sensors

Analog Devices 

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 16, a membrane ON/OFF switch is used to control the

operation of the reference. During an initial power-on condi-

tion, the SLEEP pin is held to GND by the 10 kΩ resistor.

Recall that this condition disables (read: three-state) the

REF19x output. When the membrane ON switch is pressed,

the SLEEP pin is momentarily pulled to VIN, enabling the

REF19x output. At this point, current through the 10 kΩ 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.

VIN

1k⍀

5%

ON

OFF

NC 1

8 NC

2

7 NC

REF19x

OUTPUT

3

6

1␮F

4

5 NC

TANT

10k⍀

Figure 16. 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 17. Full time current limit-

ing is used for protection of the pass transistor against shorts.

+VS = 6 TO 9V

(SEE TEXT)

C2

100␮F/25V

VC

VS

COMMON

R4

2⍀

Q2

2N3906

Q1

TIP32A

(SEE TEXT)

R1

1k⍀

R2

1.5k⍀

D1

1N4148

(SEE TEXT

ON SLEEP)

U1

REF196

(SEE TABLE)

R3

1.82k⍀

C3

0.1␮F

F

S

C1

10␮F/25V

(TANTALUM)

S

F

OUTPUT TABLE

U1 VOUT (V)

REF192 2.5

REF193 3.0

REF196 3.3

REF194 4.5

REF195 5.0

+VOUT

3.3V

@ 150mA

R1

VOUT

COMMON

Figure 17. 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–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 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 volt-

age circuit is raised about 1.1 V over that of the REF19x de-

vices, 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 maxi-

mum 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 R 4.

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

additional 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.

–18–

REV. D

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