adr391 Analog Devices, Inc., adr391 Datasheet - Page 18

• Manufacturer Part Number: adr391
• Description: Micropower, Low Noise Precision Voltage References With Shutdown
• Manufacturer: Analog Devices, Inc.

ADR390/ADR391/ADR392/ADR395
General-Purpose Current Source
Many times in low power applications, the need arises for a
precision current source that can operate on low supply vol-
tages. ADR390/ADR391/ADR392/ADR395 can be configured
as a precision current source. As shown in Figure 45, the circuit
configuration is a floating current source with a grounded load.
The reference’s output voltage is bootstrapped across R
which sets the output current into the load. With this
configuration, circuit precision is maintained for load currents
in the range from the reference’s supply current, typically 90 µA
to approximately 5 mA.
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 shown in Figure 45 provides high current without
compromising 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.
V
ADR39x
IN
Figure 44. A General-Purpose Current Source
SHDN
GND
V
IN
V
V
OUT
OUT
I
SY
(I
SET
)
ADJUST
0.1 µ F
I
SY
R1
R
L
I
I
SET
OUT
= I
R1
P1
SET
+ I
R
SET
SY
(I
SET
SET
)
,
Rev. F | Page 18 of 20
The transistor Q2 protects Q1 during short-circuit limit faults
by robbing its base drive. The maximum current is
A similar circuit function can also be achieved with the
Darlington transistor configuration, as shown in Figure 46.
V
I
Figure 45. ADR39x for High Power Performance with Current Limit
LMAX
V
IN
IN
≈ 0.6 V/R
R1
4.7k Ω
R1
4.7k Ω
Figure 46. ADR39x for High Output Current
SHDN
V
V
V
V
IN
OUT (SENSE)
with Darlington Drive Configuration
SHDN
ADR39x
V
IN
OUT (SENSE)
ADR39x
V
OUT (FORCE)
S
OUT (FORCE)
U1
U1
GND
GND
Q2N2222
Q2N2222
Q2
R
S
Q1
R
R
S
Q2
L
R
Q1
L
Q2N4921
Q2N4921
I
L