Datasheet LT3045-1 (Analog Devices) - 19

LT3045-1
APPLICATIONS INFORMATION
a ceramic output capacitor. Similarly, due to LT3045-1’s by 0.02" reduce the overall inductance to about one-fifth ultrahigh PSRR, negligible output noise is generated of a single wire. using a ceramic input capacitor. Nonetheless, given the If a battery mounted in close proximity powers the LT3045- high SET pin impedance, any piezoelectric response 1, a 4.7µF input capacitor suffices for stability. However, from a ceramic SET pin capacitor generates significant if a distantly located supply powers the LT3045-1, use a output noise – peak-to-peak excursions of hundreds of larger value input capacitor. Use a rough guideline of 1µF µVs. However, due to the SET pin capacitor’s high ESR (in addition to the 4.7µF minimum) per 6" of wire length. and ESL tolerance, any non-piezoelectrically responsive The minimum input capacitance needed to stabilize the (tantalum, electrolytic, or film) capacitor can be used at application also varies with the output capacitance as the SET pin – although electrolytic capacitors tend to well as the load current. Placing additional capacitance have high 1/f noise. In any case, use of a surface mount on the LT3045-1’s output helps. However, this requires capacitor is highly recommended. significantly more capacitance compared to additional input
Stability and Input Capacitance
bypassing. Series resistance between the supply and the LT3045-1 input also helps stabilize the application; as little The LT3045-1 is stable with a minimum 4.7µF IN pin capaci- as 0.1Ω to 0.5Ω suffices. This impedance dampens the tor. LTC recommends using low ESR ceramic capacitors. LC tank circuit at the expense of dropout voltage. A better In cases where long wires connect the power supply to alternative is to use a higher ESR tantalum or electrolytic the LT3045-1’s input and ground terminals, the use of low capacitor at the LT3045-1 input in parallel with a 4.7µF value input capacitors combined with a large load current ceramic capacitor. can result in instability. The resonant LC tank circuit formed by the wire inductance and the input capacitor is the cause
PSRR and Input Capacitance
and not because of LT3045-1’s instability. For applications utilizing the LT3045-1 for post-regulating The self-inductance, or isolated inductance, of a wire switching converters, placing a capacitor directly at the is directly proportional to its length. The wire diameter, LT3045-1 input results in AC current (at the switching however, has less influence on its self-inductance. For frequency) to flow near the LT3045-1. This relatively high- example, the self-inductance of a 2-AWG isolated wire frequency switching current generates a magnetic field with a diameter of 0.26" is about half the inductance of a that couples to the LT3045-1 output, thereby degrading its 30-AWG wire with a diameter of 0.01". One foot of 30-AWG effective PSRR. While highly dependent on the PCB, the wire has 465nH of self-inductance. switching pre-regulator, the input capacitance, amongst Several methods exist to reduce a wire’s self-inductance. other factors, the PSRR degradation can be easily over One method divides the current flowing towards the 30dB at 1MHz. This degradation is present even if the LT3045-1 between two parallel conductors. In this case, LT3045-1 is de-soldered from the board, because it ef- placing the wires further apart reduces the inductance; up fectively degrades the PSRR of the PC board itself. While to a 50% reduction when placed only a few inches apart. negligible for conventional low PSRR LDOs, LT3045-1’s Splitting the wires connect two equal inductors in parallel. ultrahigh PSRR requires careful attention to higher order However, when placed in close proximity to each other, parasitics in order to extract the full performance offered their mutual inductance adds to the overall self inductance by the regulator. of the wires — therefore a 50% reduction is not possible To mitigate the flow of high-frequency switching current in such cases. The second and more effective technique near the LT3045-1, the LT3045-1 input capacitor can be to reduce the overall inductance is to place the forward entirely removed -- as long as the switching converter’s and return current conductors (the input and ground output capacitor is located more than an inch away from wires) in close proximity. Two 30-AWG wires separated the LT3045-1. Magnetic coupling rapidly decreases with increasing distance. Nonetheless, if the switching pre- 30451fa For more information www.linear.com/LT3045-1 19 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts