Datasheet

AD5235 Data Sheet
Rev. F | Page 26 of 32
PROGRAMMABLE CURRENT SOURCE
A programmable current source can be implemented with the
circuit shown in Figure 54.
V+
V–
OP1177
U2
V
S
SLEEP
REF191
GND
OUTPUT
3
2
4
6
U1
C1
1µF
AD5235
W
A
B
R
S
102Ω
R
L
100Ω
V
L
I
L
+5V
–2.048V TO V
L
–5V
0V TO (2.048V + V
L
)
+5V
+
–
02861-053
Figure 54. Programmable Current Source
The REF191 is a unique low supply headroom and high current
handling precision reference that can deliver 20 mA at 2.048 V.
The load current is simply the voltage across Terminal W to
Terminal B of the digital potentiometer divided by R
S
.
1024×
×
=
S
REF
L
R
DV
I
(7)
The circuit is simple but be aware that there are two issues.
First, dual-supply op amps are ideal because the ground potential
of REF191 can swing from −2.048 V at zero scale to V
L
at full
scale of the potentiometer setting. Although the circuit works
under single supply, the programmable resolution of the system
is reduced by half. Second, the voltage compliance at V
L
is
limited to 2.5 V, or equivalently, a 125 Ω load. When higher
voltage compliance is needed, consider digital potentiometers,
such as, AD5260, AD5280, and AD7376. Figure 55 shows an
alternate circuit for high voltage compliance.
To achieve higher current, such as when driving a high power
LED, replace U1 with an LDO, reduce R
S
, and add a resistor in
series with the A terminal of the digital potentiometer. This
limits the current of the potentiometer and increases the current
adjustment resolution.
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution (see Figure 55). If the resistors are matched, the load
current is
WL
V
R2B
R1
R2BR2A
I ×
+
=
(8)
–15V
OP2177
V+
V–
+15V
+
–
C1
10pF
R2
15k
Ω
R1
150k
Ω
R2B
50
Ω
R
L
500
Ω
V
L
R2A
14.95k
Ω
R1
150k
Ω
I
L
OP2177
V+
V–
+15V
+
–
–15V
A1
AD5235
A
B
W
+2.5V
–2.5V
A2
02816-054
Figure 55. Programmable Bidirectional Current Source
R2B, in theory, can be made as small as necessary to achieve the
current needed within the A2 output current driving capability.
In this circuit, OP2177 delivers ±5 mA in either direction, and
the voltage compliance approaches 15 V. Without the additions
of C1 and C2, the output impedance (looking into V
L
) can be
Z
O
=
)(
)(
R2BR2AR1'R2'R1
R2AR1R2BR1'
+−
+
(9)
Z
O
can be infinite, if Resistors R1' and R2' match precisely with
R1 and R2A + R2B,
respectively, which is desirable. On the other
hand, if the resistors do not match, Z
O
can be negative and cause
oscillation. As a result, C1, in the range of a few picofarad, is
needed to prevent oscillation from the negative impedance.