PSRR Measurement of LDO

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PSRR Measurement of LDO [Copy link]

What is PSRR?

PSRR (Power supply rejection ratio), also known as power supply rejection ratio, is an important parameter to measure the circuit's ability to suppress ripple in the input power supply. It is expressed as the logarithmic ratio of the output ripple to the input ripple in decibels (dB) ^[1] . Its calculation formula is:

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Where:

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查看详情: Peak-to-peak value of ripple in output voltage

From the formula, we can see that the larger the PSRR, the smaller the ripple at the output end for the same input ripple. For RF and wireless applications with high ripple requirements, a high PSRR LDO is required. So how do you measure the PSRR of an LDO? This article summarizes various measurement methods.

PSRR Measurement Principle

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_{An AC voltage V in_AC} with a certain frequency and a peak-to-peak value of Ripple _{input (the peak-to-peak value of AC voltage is generally hundreds of millivolts) is superimposed on the DC voltage V in_DC} of the LDO input . Then, the peak-to-peak value of the AC voltage V _{out_AC Ripple output of the LDO output voltage V out_DC} is measured . Finally, the PSRR at this frequency is calculated using Formula 1.

The input voltage of the LDO needs to meet the following conditions during testing:

1. The maximum input voltage cannot exceed the maximum operating voltage of the LDO.
2. The minimum input voltage is greater than the sum of the LDO output voltage and the voltage drop.

The principle of PSRR measurement is very simple, but it is not easy to measure in practice, mainly due to the following reasons:

1. How to superimpose AC voltage on DC voltage? A signal generator with bias voltage function seems to meet the requirement, but the maximum output current of a signal generator is generally tens of milliamperes, which cannot meet the requirement if the LP5907 with an output of 150mA is to be measured .
2. How to measure the peak-to-peak value of the AC voltage in the LDO output voltage? A general oscilloscope can only measure millivolt voltage. When the PSRR of the LDO is 60dB, the output ripple is usually less than 1mV, and the oscilloscope cannot measure it accurately.

In response to the above two problems, this article will introduce the corresponding solutions.

Input DC voltage superimposed on AC voltage

1. Input Injector

Use a professional input injector, such as J2120A, with a bandwidth of 10Hz-10MHz, a maximum DC voltage of 50V, and an output current of up to 5A. Use a network analyzer to measure the AC voltage of the LDO input and output respectively, and use software to plot the PSRR of the LDO within the set frequency range.

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Figure 1 Input injector and network analyzer testing PSRR

2. Adding amplifier circuit

Use an operational amplifier to design an addition circuit to superimpose the DC voltage and AC voltage at the output. The selection of the operational amplifier needs to meet the following basic conditions:

1) The bandwidth of the op amp meets the LDO test range.

2) The maximum output current of the op amp should not be less than the maximum output current of the LDO.

3) The output voltage range of the op amp covers the input voltage range of the LDO.

TI has many operational amplifiers that meet the above requirements, such as OPA552 , OPA564 , THS3120 , etc. The addition circuit diagram is shown in Figure 2 (R1=R2). The lowest cutoff frequency of the circuit is determined by C1 and R1 ^[2] , and the highest cutoff frequency is determined by the bandwidth of the op amp.

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Figure 2 Adding op amp circuit

If the maximum DC bias voltage of the signal generator meets the measurement requirements, the operational amplifier can also be designed as a voltage follower. When measuring PSRR using this method, the input capacitor of the LDO must be removed to avoid instability of the operational amplifier.

3. LC node method

The method of realizing the superposition of DC voltage and AC voltage by using inductance and capacitance is shown in FIG3 . The highest frequency of the circuit is determined by L1 and C1 , and the lowest frequency is determined by C1 .

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Figure 3 LC node method

LDO output AC voltage measurement

1. Oscilloscope measurement

A general oscilloscope can measure millivolt-level voltages. When the PSRR of the LDO is no higher than 40dB~50dB, if the peak-to-peak value of the input AC voltage is 1V, then the peak-to-peak value of the AC voltage of the same frequency in the LDO output is 3mV~10mV, which can be directly measured with an oscilloscope.

2. Amplifier and oscilloscope measurement

When the PSRR of the LDO is greater than 50dB, the output ripple amplitude is usually less than 1mV and cannot be directly measured using an oscilloscope. In this case, you can consider using an operational amplifier to amplify the LDO output AC voltage by 100 times or even higher. When designing the operational amplifier, you need to consider:

1) The LDO output has a DC voltage, and the circuit needs to remove the DC voltage.

2) The noise generated by the amplifier circuit itself is much smaller than the amplified AC voltage.

3) The input offset voltage of the op amp cannot be too large, otherwise it will output a large DC voltage after being amplified by the amplifier circuit.

4) The bandwidth of the amplifier circuit meets the PSRR measurement frequency range of the LDO.

Therefore, when designing, you can choose an operational amplifier with low noise, low input offset voltage and high bandwidth, such as OPA211 , OPA228 , OPA189 , etc. The amplifier circuit is shown in Figure 4. The lowest cutoff frequency of the circuit is determined by C1 and R1, and the highest cutoff frequency is determined by the bandwidth of the operational amplifier.

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Figure 4 Amplifier circuit

3. Spectrum analyzer measurement

Spectrum analyzers can measure microvolt voltage signals and can be used with high-impedance input probes to measure the LDO output AC voltage. However, high-impedance input probes for spectrum analyzers are usually expensive and are not usually available in laboratories. In this case, you can consider using an operational amplifier to build a high-input impedance probe. You can refer to the circuit mentioned by Steve Hageman in Extending the Usable Range of High-Impedance FET Probes for RF Spectrum Analyzers ^[3] , as shown in Figure 5. The operational amplifier can be OPA656 .

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Figure 5 High impedance probe circuit

PSRR Measurement

The LDO measured this time is TPS7A4901. The output voltage of TPS7A4901EVM is redesigned to 1.2V, and the output capacitor is changed to 10uF. THS3120 is used as the DC voltage and AC voltage superposition circuit. THS3120EVM is used and changed to the circuit shown in Figure 6. OPA211 is selected to design the 100-fold amplification circuit shown in Figure 7.

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Figure 6 THS3120 DC voltage and AC voltage superposition circuit

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Figure 7 OPA211 amplifier circuit

The supply voltage of THS3120 and OPA211 is ±15V, the DC voltage of THS3120 is 3.2V, the AC sinusoidal voltage is 1kHz and the peak-to-peak value is 1V. The output current of TPS7A4901 is 150mA, and the NR/SS pin capacitor and feedforward capacitor are not connected. Figure 8 shows the output ripple and input ripple of LDO after amplification, and Figure 9 shows the FFT transformation of the output ripple of TPS7A4901 after amplification.

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Figure 8 LDO output ripple (yellow line) and input ripple (blue line) after amplification

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Figure 9 FFT transformation of the output ripple of TPS7A4901 after amplification

From Figure 9, we can see that the output ripple amplitude at 1kHz is -26.46dbV, which is converted into a 1kHz ripple peak-to-peak value of 0.95mV in the unamplified LDO output voltage. Using formula 1, we can get the PSRR to be 60.4dB, which is close to 62dB in the datasheet. By changing the AC voltage frequency, we can also measure the PSRR at different frequencies.

If you use an input injector and a network analyzer, you can easily measure the PSRR curve of the LDO in the set frequency range. If you don't have an input injector and a network analyzer, you can choose a combination of input and output listed above, then set a frequency, measure the AC voltage amplitude and the input and output voltage, use formula 1 to get the PSRR, then change the input AC signal frequency and repeat the measurement, and finally get the PSRR curve in the entire frequency range.