Datasheet ADR280 (Analog Devices) - 8

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ADR280 APPLICATIONS INFORMATION
The ADR280 should be decoupled with a 0.1 μF ceramic
U1
capacitor at the output for optimum stability. It is also good
5V V+
practice to include 0.1 μF ceramic capacitors at the IC supply
ADR280
pin. These capacitors should be mounted close to their
5V V–
respective pins (see Figure 12).
1.2V U2 + VO V+ VOUT V+ 1.8V AD8541 C1 0.1µF V– 0.1µF ADR280 C2 – R2 2.2pF V– 60kΩ ± 0.1% R1
7
VOUT 120kΩ ± 0.1%
01 5-
0.1µF
14 06 0 03 5- 06 Figure 15. 1.8 V Reference 03 Figure 12. Basic Configuration
LOW COST, LOW POWER CURRENT SOURCE
The low supply voltage input pin V− can be elevated above Because of its low power characteristics, the ADR280 can be ground; a 1.2 V differential voltage can therefore be established converted to a current source with just a setting resistor. In above V− (see Figure 13). addition to the ADR280 current capability, the supply voltage and the load limit the maximum current. The circuit in Figure 16
5V V+ 0.1µF
produces 100 μA with 2 V compliance at a 5 V supply. The load
ADR280
current is the sum of ISET and IGND. IGND increases slightly with
V–
load; a RSET of 13.6 kΩ yields 100 μA of load current.
2.5V U1 1.2V + V– VOUT 5V V+ 0.1µF
5 -01 5
ADR280
06 03 Figure 13. Floating References
V– ISET
The ADR280 provides the core 1.2 V band gap voltage and is
V
able to drive a maximum load of only 100 μA. Users can simply
OUT C1 + RSET 0.1µF
buffer the output for high current or sink/source current applica-
1.2V – 13.6kΩ
tions, such as ADC or LCD driver references (see Figure 14).
RL IL IGND
18
U1 1kΩ 100µA I
0
L = ISET + IGND
5- 06
V+
03
0.1µF
Figure 16. Low Cost Current Source
ADR280 V– Precision Low Power Current Source
By adding a buffer to redirect the IGND in Figure 17, a current
U2 V
can be precisely set by R
OUT
SET with the equation IL = 1.2 V/RSET.
VOUT 0.1µF U1 U2 = AD8541, SC70
6 01
5V V+ AD8601, SOT-23-5
5- 06 03
ADR280
Figure 14. Buffered Output
V–
Users can also tailor any specific need for voltage and dynamics with an external op amp and discrete components (see Figure 14
VOUT
and Figure 15). Depending on the specific op amp and PCB
C2
layout, it may be necessary to add a compensation capacitor, C2,
0.1µF
to prevent gain peaking and oscillation. The exact value of C2
5V R
needed requires some trial and error but usually falls in the
SET 12kΩ – U2 V+
range of a few picofarads.
AD8541 V– + RL I 1kΩ L 100µA
19 0 5-
IL = 1.2V/RSET
06 03 Figure 17. Precision Low Power Current Source Rev. C | Page 8 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS INFORMATION LOW COST, LOW POWER CURRENT SOURCE Precision Low Power Current Source Boosted Current Source Negative Reference Boosted Reference with Scalable Output GSM and 3G Mobile Station Applications OUTLINE DIMENSIONS ORDERING GUIDE