REF193 AD [Analog Devices], REF193 Datasheet - Page 18

• Manufacturer Part Number: REF193
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: AD [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 3,
a membrane 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 kΩ 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
V
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 V
the SLEEP pin is momentarily connected to GND, which once
again disables the REF19x output.
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.
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
+V
Figure 4. A Boosted 3.3 V Reference with Current Limiting
IN
S
Figure 3. Membrane Switch Controlled Power Supply
= 6V TO 9V
, enabling the REF19x output. At this point, current through
COMMON
(SEE TEXT)
100 F/25V
V
V
OFF
C
ON
S
1k
V
C2
5%
IN
1N4148
(SEE TEXT
ON SLEEP)
2N3906
D1
IN
Q2
. When the membrane OFF switch is pressed,
NC
R4
2
NC = NO CONNECT
(SEE TABLE)
REF196
1
2
3
4
U1
REF19x
10k
R1
1k
R2
1.5k
1.82k
(SEE TEXT)
TIP32A
R3
Q1
8
7
6
5
(TANTALUM)
C3
0.1 F
NC
NC
NC
10 F/25V
OUTPUT
S
F
S
C1
F
1 F
TANT
REF192
REF193
REF196
REF194
REF195
OUTPUT TABLE
R1
U1
V
COMMON
+V
3.3V
@ 150mA
OUT
OUT
V
OUT
2.5
3.0
3.3
4.5
5.0
(V)
–18–
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 V
“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 V
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 V
of 150 mA.
The requirement for a heat sink on Q1 depends on the maximum
input voltage and short circuit current. With V
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 V
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 V
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.
BE
of Q1 and the drop across current sense resistor R4.
S
as low as 4.5 V and a load current
S
line to U1, direct drive to Pin 3
OUT
S
= 5 V and a
sense point
C
to drive
REV. E