ref195gru-reel Analog Devices, Inc., ref195gru-reel Datasheet - Page 20

• Manufacturer Part Number: ref195gru-reel
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: Analog Devices, Inc.

REF19x Series
CURRENT-BOOSTED REFERENCES WITH
CURRENT LIMITING
While the 30 mA rated output current of the REF19x series is
higher than is 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 23. Full-time current
limiting is used to protect the pass transistor against shorts.
(SEE TEXT)
COMMON
In this circuit, the power supply current of reference U1 flowing
through R1 to 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 tempera-
ture-related drift. Short-circuit protection is provided by Q2,
which clamps the drive to Q1 at about 300 mA of load current,
with values as shown in Figure 23. With this separation of
control and power functions, dc stability is optimum, allowing
most advantageous use of premium grade REF19x devices for
U1. Of course, load management should still be exercised. A
short, heavy, low dc resistance (DCR) conductor should be used
from U1 to U6 to the V
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
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 V
a load current of 150 mA.
V
+V
C
TO 9V
V
S
100µF
S
= 6V
25V
C2
OFF
Figure 23. Boosted 3.3 V Referenced with Current Limiting
ON
V
1kΩ
5%
+
Figure 22. Membrane Switch-Controlled Power Supply
1N4148
(SEE TEXT
ON SLEEP)
IN
2N3906
D1
Q2
3
2Ω
R4
(SEE TABLE)
REF196
BE
NC
U1
NC = NO CONNECT
of Q1 and the drop across Current Sense
2
4
OUT
R1
1kΩ
R2
1.5kΩ
1
2
3
4
1.82kΩ
REF19x
Sense Point S, where the collector of
(SEE TEXT)
10kΩ
6
TIP32A
R3
Q1
(TANTALUM)
C3
0.1µF
8
7
6
5
10µF/25V
NC
NC
OUTPUT
NC
S
C1
S
F
F
S
+
as low as 4.5 V and
U1
REF192
REF193
REF196
REF194
REF195
R1
+V
3.3V
@ 150mA
V
COMMON
OUTPUT TABLE
OUT
+
OUT
1µF
TANT
V
2.5
3.0
3.3
4.5
5.0
OUT
(V)
Rev. I | Page 20 of 28
The requirement for a heat sink on Q1 depends on the maximum
input voltage and short-circuit current. With V
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 R4.
A tantalum output capacitor is used at C1 for its low equivalent
series resistance (ESR), 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 an option, and when
used, some cautions are needed. Due to the additional active
devices in the V
work as with an unbuffered REF19x device. To enable shutdown
control, the connection from U1 to U2 is broken at the X, and
Diode D1 then allows a CMOS control source, V
to U3 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.
NEGATIVE PRECISION REFERENCE WITHOUT
PRECISION RESISTORS
In many current-output CMOS DAC applications where the
output signal voltage must be the same polarity as the reference
voltage, it is often necessary to reconfigure a current-switching
DAC into a voltage-switching DAC using 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, because 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 using an operational
amplifier and a pair of matched resistors in an inverting
configuration. The disadvantage to this approach is that the
largest single source of error in the circuit is the relative
matching of the resistors used.
The circuit illustrated in Figure 24 avoids the need for tightly
matched resistors by using an active integrator circuit. In this
circuit, the output of the voltage reference provides the input
drive for the integrator. To maintain circuit equilibrium, the
integrator adjusts its output to establish the proper relationship
between the reference’s V
negative output voltage can be selected by substituting for the
appropriate reference IC. The sleep feature is maintained in the
circuit with the simple addition of a PNP transistor and a 10 kΩ
resistor.
S
line to U1, a direct drive to Pin 3 does not
OUT
and GND. Thus, any desired
S
= 5 V and a 300
C
, to drive U1