The hottest linear regulator with high power rejec

2022-08-14
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Linear regulator with high power rejection ratio

low voltage drop regulator (LDO) is mainly used to generate low ripple and low noise power supply for audio and RF circuits, and can also be used as local pure power supply for frequency synthesizer and VCO. Generally, the input of LDO is the power supply voltage with broadband AC ripple superimposed on the DC voltage. The electrical noise caused by the change of current flowing through the battery and connector impedance is one of the broadband AC ripple. LDO can be used to suppress these parasitic signals comprehensively. The input of LDO can also be connected to the output of switching regulator to provide pure and low-noise output voltage. In this application, LDO must be able to cope with switching frequencies from 100kHz to 3MHz or above

first, LDO regulator can be regarded as a low loss, compact multi section low-pass filter with switching function. However, this model has many limitations, especially the ability to suppress the input broadband ripple. Assume that the working current is less than 250 μ A (for reasonable efficiency), IC designers can optimize some main performances, such as noise, regulation and power rejection ratio, within the limit of gain bandwidth caused by low quiescent current. Further performance improvements require other measures

when high-performance power supply is required, designers should start by adding as many functions as possible to the regulator. The following will take the output noise as 9 μ As components with VRMs and 80dB power supply rejection ratio at 10kHz are taken as examples to illustrate LDO design examples

power supply suppression

in practical applications, non ideal components and parasitic capacitors will change the ideal suppression characteristics of linear regulators. Figure 1 shows some significant defects that change or reduce the suppression ability of the circuit

Figure 1: p-channel LDO shows the simplified parasitic high-frequency channel

linear regulation rate

LDO data manual has two indicators to describe the ability of LDO to suppress input voltage noise, respectively, linear regulation rate and power supply rejection ratio (PSRR). Although they look very similar, one indicator reflects the DC change, and the other indicator reflects the AC performance

the linear regulation rate represents the ability of LDO to suppress the change of input voltage. The following formula is expressed:

in practical application, the linear regulation rate can be regarded as the output voltage Vout of the regulator. With the quality, it can help the product to show the percentage of the change of input voltage Vin per volt. This is particularly useful when the same regulator has various output voltage fine-tuning functions

linear regulation rate is a steady-state DC parameter, which is mainly determined by the open-loop gain of the regulator at the zero frequency point

power supply rejection ratio

this index measures the ability of the regulator to suppress the AC signal superimposed on the normal input DC voltage

the power supply rejection ratio is the largest at low frequency, and it begins to decline from 1kHz to 10kHz according to the actual regulator design. Figure 2 shows the typical PSRR characteristics of as 150ma/300ma LDO with low noise and high power rejection ratio. This component still has a good PSRR value of 60dB at 100kHz

Figure 2: the power rejection ratio of as components

the curve shown in Figure 2 is formed by many dominant influences depending on the frequency of interest. In the range of DC and close to 100Hz to 1kHz, the suppression effect depends on the bandgap reference and the open-loop gain of the regulator error signal amplifier. Above this range, up to about 100kHz, the suppression effect mainly depends on the open-loop gain of the error signal amplifier. However, above 100kHz, the power supply rejection ratio is mainly affected by the output capacitance, parasitic components, and any leakage current and packaging on the regulator. Figure 1 shows these components

if there are additional power supply suppression requirements for frequencies above 100kHz (usually also required), an external pre filter network must be connected at the input to enhance the suppression performance of LDO

external network

in practical applications, only two pre filtering methods are worth considering, because excessive power consumption on additional components must be prevented, while maintaining the stability of the regulator

Figure 3: Series LDO can produce high PSRR

Figure 4: external input network can provide additional suppression performance

method 1 - add a linear regulator. This method is very simple, only needs to occupy a small amount of PCB area (for example, as requires 2 TSOT packages), and the design time is the shortest compared with other methods. By connecting two linear regulators in series, the PSRR at any frequency point can be doubled (assuming that the two regulators are identical). The disadvantage of this method is that the voltage drop will double and an additional capacitor is required. The typical voltage drop of two as1359 in series is 280mv at 300mA load current and the rejection ratio is 120dB at 100kHz. To achieve this level, the layout of components and PCB routing must be carefully handled

from a practical point of view, a voltage drop of 350mV to 400mV should be allowed on each LDO. The output voltage range of as LDO regulator is 1.5V to 4.5V, and the adjustment step is 50mV, so the special voltage is not a problem

method 2 - LC filter based on unified dissipative network is loaded at the input. The three pole Butterworth characteristic requires a series of low-value lines, which fully demonstrates the good heritage of PPG in the field of composite materials and two capacitors, one of which is usually located at the input end of LDO. The three pole Butterworth response can provide an additional 40dB of suppression at 4.5fo, where fo is the -3db point (see Figure 5)

Figure 5: Butterworth stopband attenuation under different order n

compared with method 1, the additional filter network will not increase the significant DC impedance loss. However, special care should be taken in the design to ensure that the selected inductor can support the required DC current without saturation. Butterworth is a very useful characteristic, in which the passband is particularly flat, which is different from the Chebyshev characteristic, which sacrifices the passband and stopband ripple in order to obtain a better attenuation rate. Figure 7 shows the attenuation characteristics of a Butterworth filter with stages between 2 and 10

Figure 6 presents the element values normalized to 1 Ω source impedance and 1rad/s frequency. Note that the filter output is not terminated with a terminal resistor, which will allow it to be connected to the high input impedance of the linear regulator. Column D contains losses caused by non ideal components and indicates passband losses. For power filter, it is not necessary to strictly conform to Butterworth characteristics; After all, the filter has only a small loss at the DC point. Figure 6a: unified dissipation value of three pole Butterworth normalized to 1 Ω and 1rad/s

figure 6B: unified dissipation filter of three pole Butterworth

the following expression is used to de normalize the value in Figure 8A:

for power application, the actual source impedance selected is race=0.1 Ω. If C2 is fixed to 1 μ F (as), then W and a must be selected repeatedly until they are close to the value of commercial components. Passband loss is not a major problem, and additional AC attenuation is very important in this application

assume c3=1 μ F. F-3db=1mhz (calc 1.082), and the characteristic=0.1 Ω, then c1=1.5 μ F (Calc 1.47), l2=38nh (d=0. a year-on-year increase of 91.11% and 146.79% respectively, a=6.82db). Suitable coils can be selected from the mini spring series (b09tjlc) or MIDI spring series (1812sms-39njlc) of coolcraft. As long as the parasitic inductance of ceramic capacitors (c1&c3) is less than 1NH, the filter has sufficient attenuation value

summary of this paper

as LDO has good noise and power suppression performance. As long as a simple LC network is added at the input, additional high-frequency power supply rejection ratio performance can be achieved. (end)

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