ADR391/ADR392/ADR395
Rev. H | Page 16 of 20
General-Purpose Current Source
Many times in low power applications, the need arises for a
precision current source that can operate on low supply voltages.
The ADR391/ADR392/ADR395 can be configured as a precision
current source. As shown in Figure 37, the circuit configuration
is a floating current source with a grounded load. The reference
output voltage is bootstrapped across R
SET
, which sets the output
current into the load. With this configuration, circuit precision
is maintained for load currents in the range from the reference
supply current, typically 90 μA to approximately 5 mA.
ADR39x
GND
R1
P1
R1
0.1µF
SHDN
IN
V
IN
V
OUT (FORCE)
V
OUT (SENSE)
I
SET
R
SET
I
OUT
= I
SET
+ I
SY
(I
SET
)
R
L
I
SY
ADJUST
I
SY
(I
SET
)
00419-044
Figure 37. A General-Purpose Current Source
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 38 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.
The Transistor Q2 protects Q1 during short-circuit limit faults
by robbing its base drive. The maximum current is
I
LMAX
≈ 0.6 V/R
S
(6)
SHDN
GND
U1
ADR39x
00419-045
V
IN
V
IN
V
OUT (FORCE)
R
L
I
L
R
S
V
OUT (SENSE)
Q2
Q2N2222
R1
4.7kΩ
Q1
Q2N4921
Figure 38. ADR39x for High Power Performance with Current Limit
A similar circuit function can also be achieved with the
Darlington transistor configuration, as shown in Figure 39.
ADR39x
GND
U1
00419-D-046
SHDN
V
IN
V
IN
V
OUT (FORCE)
R
L
R
S
V
OUT (SENSE)
Q1
Q2N2222
R1
4.7kΩ
Q2
Q2N4921
Figure 39. ADR39x for High Output Current
with Darlington Drive Configuration
