Datasheet ADR280 (Analog Devices) - 9

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ADR280 Boosted Current Source
100 mA load is achievable at a 5 V supply. The higher the Adding one more buffer to the previous circuit boosts the supply voltage, the lower the current handling is because of the current to the level that is limited only by the buffer U2 current heat generated on the MOSFET. For heavy capacitive loads, handling capability (see Figure 18). additional buffering is needed at the output to enhance the transient response.
U1 5V U1 5V V+ 2.5V/100mA VO V+ ADR280 RL ADR280 V– 25Ω M1* 5V V– U2 + VREF V+ VOUT 1.2V R2 U2 + V+ VOUT V– – C1 C2 10.8kΩ ± 0.1% 1pF AD8541 0.1µF V– C1 – 0.1µF 5V + RSET – U3 1.2V 230Ω V+ – R1 *M1 = FDB301N, 2N7000, 2N7002, OR EQUIVALENT. V–
22
+ 10kΩ ± 0.1%
0 5-
RL IL
06
500Ω 5mA
03 0 Figure 20. 2.5 V Boosted Reference
I
02
L = 1.2V/RSET
5-
U2 = U3 = AD8542, AD822
06 03
GSM and 3G Mobile Station Applications
Figure 18. Precision Current Source The ADR280 voltage reference is ideal for use with analog
Negative Reference
baseband ICs in GSM and 3G mobile station applications. A negative reference can be precisely configured without using Figure 21 illustrates the use of the ADR280 with the AD6535 any expensive tight tolerance resistors, as shown in Figure 19. GSM analog baseband. The AD6535 provides all of the data The voltage difference between V converters and power management functions needed to OUT and V− is 1.2 V. Since VOUT is at virtual ground, U2 closes the loop by forcing the V− pin to implement a GSM mobile station, including baseband codecs, be the negative reference output. audio codecs, voltage regulators, and a battery charger. Besides
U1
low current consumption and a small footprint, the ADR280 is optimized for excellent PSRR, which is necessary for optimum
+5V V+
AD6535 device performance when the main battery voltage
ADR280
fluctuates during RF power amplifier activity.
V– –VREF DIGITAL AD6535 RADIO –1.2V BASEBAND ANALOG BASEBAND VOUT C1 BASEBAND CODEC 0.1µF U2 – V+ AD8541 AUDIO CODEC V– +
021 65-
–2.7V POWER
30 0
MANAGEMENT
Figure 19. Negative Reference
Boosted Reference with Scalable Output
3 A precision user defined output with boosted current capability
ADR280
02 5-
VOLTAGE REFERENCE
06 can be implemented with the circuit shown in Figure 20. In this 03 Figure 21. GSM Mobile Station Application circuit, U2 forces VO to be equal to VREF × (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 Rev. C | Page 9 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