Maxim > App Notes > Filter circuits (analog)Power-supply circuits
Keywords: power supply rejection ratio, PSRR, low drop out regulator, LDOs, linear regulators, dampening, filter, noi, voltage control
Oct 01, 2002 oscillator, VCO, line regulation
APPLICATION NOTE 883
Improved power supply rejection for IC linear regulators
Abstract: In portable communications low-dropout linear regulators (LDOs) are ud to generate supply voltages for the RF circuitry; the voltages must be especially clean when powering the synthesizer and voltage-controlled oscillator (VCO). The supply that powers the regulator often includes wideband AC ripple superimpod on the DC. The LDO is expected to reject
the artifacts. This article prents three methods for improving the power supply rejection ratio (PSRR) for LDOs.
The design of integrated linear regulators for battery applications is full of difficult compromis. Designs must deliver low operating current for long-battery life while supplying clean, well-regulated power in a sometimes noisy environment. This note describes techniques for improving the rejection of AC artifacts while maintaining low operating current.
Challenges
Regulator operating currents of less than 250µA limit the available gain-bandwidth, making specifications such as noi, regulation, and power supply rejection tricky to achieve.
In portable communications, low-dropout linear regulators (LDOs) are ud to generate supply voltages for the RF circuitry; the voltages must be especially clean when powering the synthesizer and VCO. The supply that powers the regulator often includes wideband AC ripple superimpod on the DC. The LDO is expected to reject the artifacts.
When an LDO is powered by a switching regulator, it must be able to cope with switching frequencies beyond 300kHz.
账面值Designers expect the capabilities without an increa in the LDO's quiescent current.
Line regulation
There are two specifications in the LDO data sheet that refer to the LDO's ability to reject the various forms of noi on the incoming supply. They are line regulation and power-supply rejection ratio (PSRR).
Line regulation measures the ability of the LDO to ignore changes of input voltage. Mathematically,
龙池小学In practice, line regulation is referred to the regulator output voltage in terms of %/V OUT. This is particularly uful when the same regulator is available with numerous output voltage trim options.
Line regulation is a steady-state DC measurement, and is a measure of the regulator open-loop current gain at zero frequency.
Power supply rejection ratio
This specification is the measure of how well the regulator rejects an AC signal riding on a nominal input DC voltage.
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Power-supply rejection ratio is at a maximum at low frequencies, and begins to fall above 1kHz to 10kHz, depending upon the regulator design. Figure 1 shows the PSRR typical characteristic for the MAX8867 150mA low-noi LDO, and Figure 2 shows the PSRR characteristic for the MAX1792 500mA LDO.
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Figure 1. MAX8867 PSRR characteristic.
Figure 2. MAX1792 PSRR characteristic.
The only way to modify the basic rejection respon of the regulator is to add an external network at the input of the regulator.
External networks
There are three methods to choo from:
1. One or more cascades of external RC filters. The additional attenuation adds to the inherent characteristic of the
regulator. The ries resistor(s) has to be kept low in order to minimize the IR loss and conquent reduction of regulator headroom. This limitation requires that large bulk capacitors be ud in association with 1Ω to 10Ω resistors. If the load current is very low, say less than 20mA, then an RC filter is uful.On the other hand, if there is plenty of voltage headroom, and space to dissipate heat, then an RC filter(s) may also be practical for larger load currents. The upper limit for ries resistance is defined by the stability of the regulator. The designer assumes a low source impedance for the input supply. A ries R greater than 200Ω is entering dangerous territory.
The low-pass-filter transfer characteristic for a single RC is
. Ultimate attenuation is given by
Its attenuation characteristic is -20dB / decade from the corner frequency f = (2π RC)-1
The transfer characteristic of a cond-order cascade low-pass filter with equal-value Rs and Cs is
Typical values for the single RC and cascade RC filters range from R=1Ω to 10Ω, and C=100µF to 10µF respectively.
Choo the network -3dB frequency to coincide with that of the regulator PSRR characteristic.
Figure 3a shows the single order RC network, and Figure 3b shows the cascade cond-order RC network, both
protecting a linear regulator.
Figure 3a. Single RC ripple filter.
2. An LC filter. The problem with using this type of filter is the lack of inherent damping at the output of the network (input of
the regulator). The source impedance of the network is low. However, the regulator's V IN terminal prents a high
impedance shunted with a small capacitor. (When the regulator is operated away from dropout with a constant load, its input current does not vary, to first order, when V IN is varied.) It is impossible to critically damp the LC network at the input of the regulator without significant DC loss in the shunt damping resistor. For example, a ries 10µH inductor in
combination with a shunt 100µF capacitor exhibits a turnover frequency of 5kHz; this combination re
quires a critical damping resistor of 0.32Ω between network output (regulator input) and ground. Figure 4a illustrates the problem.
Figure 4a. LC filter amplitude characteristic for various damping ratios.
Figure 4b shows the damping resistor and its relationship with the noisy incoming supply source resistance.
Figure 4b. Illustrating the position of damping resistor.
3. An additional linear regulator. This method occupies a small PCB area (e.g., two SOT23-5 packages) and requires
the least design time of the other methods. Using two linear regulators in ries doubles the PSRR at any given frequency (assuming identical regulators). The "penalty" for this approach is the doubling of the dropout voltage and the need for an additional capacitor. A good design choice is to share the voltage drop across each regulator. Two MAX8867 regulators in ries provide at least 80dB of PSRR at 100kHz, and the total asmbly requires three 1µF ceramic capacitors, one each at the input, output, and the intermediate position. Figure 5 shows two linear regulators cascaded in t
his manner.
Figure 5. Series cascade of LDOs for input ripple isolation.
A MAX8875 (LDO1) followed by a MAX8867 (LDO2) gives a low-noi output and 70d
B of PSRR at 100kHz. All capacitors
are 1µF.
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High-output-current regulator protection
The previous examples and techniques have focud on low-current single-package LDOs. An RC filter may be added to a high-output-current LDO and produce excellent results with little or no additional IR drop. For this high-current LDO, thermal constraints place the ries power transistor o
utside the main control IC. The supply to the control IC is a fraction of the main current path. Therefore the control-IC power supply is an ideal place to add RC ripple filtering. Figure 6 shows an example employing the MAX687-9 family of 4A LDO regulator controllers.
Figure 6. Additional protection for high current LDO.
The supply current at the IN terminal of the MAX687-9 is 250µA. The addition of R1 provides additional filtering to the whole control circuit and enhances the overall circuit PSRR to better than 60dB at 100kHz.
Conclusion
箫教程An external network added to the input of a linear regulator improves the inherent PSRR of the LDO, especially at high frequencies, where the low quiescent current compromis the high-frequency PSRR of the LDO.
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日慎一日Of the methods outlined above, the additional LDO is the most general purpo, packs more attenuation into a given small area, and requires the least design time.
For low-current applications requiring only modest protection, the RC filter method is cost competitive, but requires careful tradeoffs in component lection.
For high-current LDO controllers, the addition of one resistor to the controller input supply very effectively enhances the PSRR of the entire circuit.
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