advertisement Lowest Noise SOT-23 LDOs Have 20µA Quiescent Current, 20µVRMS Noise – Design Note 220 Todd Owen and Jim Williams Applying the Regulators Applying the regulators is simple. Figure 1 shows a minimum parts count, 3.3V output design. This circuit appears similar to conventional approaches with a 1μF OUT 0.01μF LT1761-3.3 SHDN L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 500 450 3.3V/100mA 20μVRMS NOISE (TYP) VIN > 3.7V IN notable exception: a bypass pin (BYP) is returned to the output via a 0.01μF capacitor. This path filters the internal reference’s output, minimizing regulator output noise. It is the key to the 20μVRMS noise performance. A shutdown pin (SHDN), when pulled low, turns off the regulator output while keeping current drain inside 1μA. Dropout characteristics appear in Figure 2. Dropout scales with output current, falling to less than 100mV at low currents. DROPOUT VOLTAGE (mV) Telecom and instrumentation applications often require a low noise voltage regulator. Frequently this requirement coincides with the need for low regulator dropout and small quiescent current. LTC recently introduced a family of devices to address this problem. Table 1 shows a variety of packages, power ranges and features in three basic regulator types. The SOT-23 packaged LT®1761 has only 20μVRMS noise with 300mV dropout at 100mA. Quiescent current is only 20μA. + BYP 10μF DN220 F01 GND 400 350 300 IL = 100mA 250 IL = 50mA 200 150 IL = 10mA 100 IL = 1mA 50 Figure 1. Applying the Low Noise, Low Dropout, Micropower Regulator. Bypass Pin and Associated Capacitor are Key to Low Noise Performance 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 DN220 F02 Figure 2. Figure 1’s Dropout Voltage at Various Currents Table 1. Low Noise LDO Family Short-Form Specifications. Quiescent Current Scales with Output Current Capability, Although Noise Performance Remains Constant OUTPUT CURRENT RMS NOISE (10Hz to 100kHz) CBYP = 0.01μF PACKAGE OPTIONS LT1761 100mA 20μV SOT-23 LT1762 150mA 20μV LT1763 500mA 20μV REGULATOR TYPE 12/99/220_conv QUIESCENT CURRENT SHUTDOWN CURRENT Shutdown, Reference Bypass, Adjustable Output. SOT-23 Package Mandates Selecting Any Two Features 20μA <1μA MS8 Shutdown, Reference Bypass, Adjustable Output 25μA <1μA SO-8 Shutdown, Reference Bypass, Adjustable Output 30μA <1μA FEATURES Noise Performance Noise performance is displayed in Figure 3. This measurement was taken in a 10Hz to 100kHz bandwidth with a “brick wall” multipole filter1. The photo’s trace, applied to a thermally responding RMS voltmeter, contains less than 20μVRMS noise. Figure 4 shows noise in the frequency domain with noise power falling with increasing frequency. Other Advantages The LT1761 family is stable (no output oscillation) even when used with low ESR ceramic output capacitors. This is in stark contrast to LDO regulators from other manufacturers that often oscillate with ceramic capacitors. The unique internal architecture provides an added bonus in transient performance when adding a 0.01μF noise capacitor. Transient response for a 10mA to 100mA step with a 10μF output capacitor is shown in Figure 5. Figure 6 shows the same setup with the addition of a 0.01μF bypass capacitor. Settling time and amplitude are markedly reduced. Conclusion These devices provide the lowest available output noise in a low dropout regulator without compromising other parameters. Their performance, ease of use and versatility allow use in a variety of noise-sensitive applications. 1Noise measurement and specification of regulators requires care and will be comprehensively treated in a forthcoming LTC Application Note. OUTPUT VOLTAGE NOISE (μV/√Hz) 10 ENOISE 100μV/DIV 1ms/DIV DN220 F03 LT1761-5 LT1761-3.3 1 LT1761 0.1 0.01 10 Figure 3. LT1761 Output Voltage Noise in a 10Hz to 100kHz Bandwidth. 20μVRMS Noise is the Lowest Available in an LDO 100 1k 10k FREQUENCY (Hz) 100k DN220 F04 Figure 4. Output Noise Spectral Density for Figure 1’s Circuit. Curves for Three Output Versions Show Dispersion Below 200Hz VOUT 20mV/DIV VOUT 100mV/DIV 200μs/DIV DN220 F05 Figure 5. Transient Response with No Noise Bypass Capacitor Data Sheet Download www.linear.com Linear Technology Corporation 200μs/DIV DN220 F06 Figure 6. Noise Bypass Capacitor Improves Transient Response. Note Change in Voltage Scale For applications help, call (408) 432-1900 dn220f_conv LT/TP 1299 340K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1999