LT3074AVPBF datasheet(36/62 Pages) AD

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Data Sheet

LT3074

analog.com

Rev 0.

36 of 62

PSRR and Input Capacitance

For applications using the LT3074 for post-regulating switching converters, placing a capacitor directly at the input

of the LT3074 results in AC current (at the switching frequency) to flow near the LT3074. The relatively high frequency

switching current generates a magnetic field that couples to the output of the LT3074, degrading its effective power

supply rejection ratio (PSRR). While highly dependent on the PCB, the switching pre-regulator, and the input

capacitance, among other factors, the PSRR degradation is present even if the LT3074 is desoldered from the board

because it effectively degrades the PSRR of the PCB itself. While negligible for conventional, low PSRR, low dropout

(LDO) regulators, the high PSRR of the LT3074 requires careful attention to higher order parasitics to extract the full

performance offered by the regulator.

The input capacitor of the LT3074 cannot be completely removed to mitigate the flow of the high frequency switching

current near the LT3074 because with no in put capacitor present, as with any regulator, the input of the LT3074

oscillates at the parasitic LC resonant frequency. In addition it is generally common (and preferred) to bypass the

regulator input with some capacitance.

To that end, Analog Devices recommends using the LT3074 evaluation board layout for achieving the best possible

PSRR performance. The LT3074 evaluation board layout uses magnetic field cancellation techniques to prevent PSRR

degradation caused by this high frequency current flow, while using an input capacitor.

Filtering High Frequency Spikes

For applications where the LT3074 is used to post regulate a switching converter, its high PSRR effectively suppresses

any noise present at the switching frequency of the switching converter, typically 100kHz to 4MHz. However, the high

frequency (hundreds of MHz) spikes, beyond the bandwidth of the LT3074, associated with the power-switch

transition times of the switching converter almost directly pass through the LT3074. While the output capacitor is

intended to partly absorb these spikes, its ESL limits its ability at these frequencies. A ferrite bead or even the

inductance associated with a short (example 0.5”) PCB trace between the output of the switching converter and the

input of LT3074 can serve as an LC filter to suppress these high frequency spikes.

Output Noise

The LT3074 offers many advantages with respect to noise performance. Traditional linear regulators have several

sources of noise. The most critical noise sources for a traditional regulator are its voltage reference, error amplifier,

noise from the resistor divider network used for setting the output voltage, and the noise gain created by this resistor

divider. Many low noise regulators pin out their voltage reference to allow for noise reduction by bypassing the

reference voltage. Unlike most linear regulators, LT3074 does not use a voltage reference. Instead, the LT3074 uses

a 100µA current reference into the SETRES pin. The resultant voltage noise equals the current noise multiplied by the

resistor value, which in turn, is the root mean square (RMS) summed with the noise of the error amplifier and the

thermal noise of the resistor.

One problem that conventional linear regulators face is that the resistor divider setting the output voltage gains up

the reference noise. In contrast, the unity-gain architecture of the LT3074 presents no gain from the SETCAP pin to

the output. In addition, the SETCAP is decoupled from the SETRES pin internally with a small value filter resistor

(approximately 1kΩ). This filter resistor, along with the SETCAP capacitor, creates a low-pass filter (LPF) bypassing

the SETRES pin resistor noise. As a result, the output noise is independent of the programmed output voltage. The

resultant output noise is then set just by the noise of the error amplifier, typically 3.5nV/√Hz from a 10kHz to 1MHz

bandwidth and 1.2µVRMS from a 10Hz to 100kHz bandwidth using a 4.7µF capacitor. Paralleling multiple LT3074

devices further reduces noise by a factor of √N for N parallel regulators.

LT3074AVPBF 数据表(PDF) 36 Page - Analog Devices

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