ADR280
Rev. C | Page 9 of 12
Boosted Current Source
Adding one more buffer to the previous circuit boosts the
current to the level that is limited only by the buffer U2 current
handling capability (see
Figure 18).
V+
V–
ADR280
U1
5V
C1
V+
V–
5V
U3
V+
V–
5V
U2
1.2V
+
–
+
–
+
–
V
OUT
0.1µF
R
SET
230Ω
RL
500Ω
I
L
5mA
I
L
= 1.2V/R
SET
U2 = U3 = AD8542, AD822
03065-020
Figure 18. Precision Current Source
Negative Reference
A negative reference can be precisely configured without using
any expensive tight tolerance resistors, as shown in
Figure 19.
The voltage difference between V
OUT
and V− is 1.2 V. Since V
OUT
is at virtual ground, U2 closes the loop by forcing the V− pin to
be the negative reference output.
V+
V–
ADR280
U1
+5V
V+
V–
–2.7V
AD8541
–1.2V
U2
–V
REF
V
OUT
–
+
C1
0.1µF
03065-021
Figure 19. Negative Reference
Boosted Reference with Scalable Output
A precision user defined output with boosted current capability
can be implemented with the circuit shown in
Figure 20. In this
circuit, U2 forces V
O
to be equal to V
REF
× (1 + R2/R1) by regu-
lating the turn-on of M1; the load current is therefore furnished
by the 5 V supply. For higher output voltage, U2 must be changed
and the supply voltage of M1 and U2 must also be elevated and
separated from the U1 input voltage. In this configuration, a
100 mA load is achievable at a 5 V supply. The higher the
supply voltage, the lower the current handling is because of the
heat generated on the MOSFET. For heavy capacitive loads,
additional buffering is needed at the output to enhance the
transient response.
*M1 = FDB301N, 2N7000, 2N7002, OR EQUIVALENT.
V+
V–
ADR280
U1
5V
V+
V–
U2
C2
1pF
V
REF
1.2V
AD8541
M1*
V
OUT
2.5V/100mA
V
O
RL
25Ω
R2
10.8kΩ ± 0.1%
R1
10kΩ ± 0.1%
C1
0.1µF
+
–
03065-022
Figure 20. 2.5 V Boosted Reference
GSM and 3G Mobile Station Applications
The ADR280 voltage reference is ideal for use with analog
baseband ICs in GSM and 3G mobile station applications.
Figure 21 illustrates the use of the ADR280 with the AD6535
GSM analog baseband. The AD6535 provides all of the data
converters and power management functions needed to
implement a GSM mobile station, including baseband codecs,
audio codecs, voltage regulators, and a battery charger. Besides
low current consumption and a small footprint, the ADR280 is
optimized for excellent PSRR, which is necessary for optimum
AD6535 device performance when the main battery voltage
fluctuates during RF power amplifier activity.
RADIO
BASEBAND CODEC
AUDIO CODEC
POWER
MANAGEMENT
ADR280
VOLTAGE REFERENCE
AD6535
ANALOG BASEBAND
DIGITAL
BASEBAND
03065-023
Figure 21. GSM Mobile Station Application
