PSRR Measurement of LDO

okhxyyo

PSRR Measurement of LDO [Copy link]

PSRR measurement of LDO

[size=1em]Author: Hao Wang Shenzhen Analog Engineer

What is PSRR

[size=1em]PSRR (Power supply rejection ratio), also known as power supply rejection ratio, is an important parameter to measure the ripple rejection of the circuit to 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:

In the formula:

: Peak-to-peak value of ripple in input voltage

: 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, LDOs with high PSRR need to be selected. So how to measure the PSRR of LDO? This article summarizes various measurement methods. PSRR measurement principle: Superimpose an AC voltage Vin_AC with a certain frequency and a peak-to-peak value of Rippleinput (the peak-to-peak value of the AC voltage is generally hundreds of millivolts) on the DC voltage Vin_DC of the LDO input, and then measure the peak-to-peak value Rippleoutput of the AC voltage Vout_AC of the LDO output voltage Vout_DC. Finally, use formula 1 to calculate the PSRR at this frequency.

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

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

The principle of PSRR measurement is very simple, but it is not easy to find in the actual measurement process, mainly reflected in:

• How to superimpose AC voltage on DC voltage? A signal generator with bias voltage function seems to meet the requirements, but the maximum output current of the signal generator is generally tens of mA. If you want to measure the LP5907 with an output of 150mA, it will not meet the requirements.
• How to measure the peak-to-peak value of AC voltage in LDO output voltage? General oscilloscopes can only measure millivolt voltages. When the PSRR of LDO is 60dB, the output ripple is usually less than 1mV, and the oscilloscope cannot measure it accurately. This article will introduce the corresponding solutions to the above two problems. 1. Input DC voltage superimposed on AC voltage 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 at the input and output of the LDO, and use software to plot the PSRR of the LDO within the set frequency range. Figure 1 Input injector and network analyzer test PSRR 2. Adder op amp circuit Use an operational amplifier to design an adder circuit to superimpose the DC voltage and AC voltage at the output. The selection of the op amp 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 is not 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.
  Figure 2 Adder 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 should be removed to avoid instability of the operational amplifier.
  3. LCNode method
  The method of using inductance and capacitance to realize the superposition of DC voltage and AC voltage is shown in Figure 3. The highest frequency of the circuit is determined by L1 and C1, and the lowest frequency is determined by C1.
  [img=383,148]https://e2echina.ti.Figure 3 LC node method LDO output AC voltage measurement 1. Oscilloscope measurement A general oscilloscope can measure millivolt level voltage. When the PSRR of the LDO is not 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 with 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 LDO is greater than 50dB, the output ripple amplitude is usually less than 1mV, so it cannot be directly measured using an oscilloscope. At this time, 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 must be much smaller than the amplified AC voltage. 3) The input offset voltage of the operational amplifier cannot be too large, otherwise it will output a large DC voltage after amplification 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. 3. Spectrum analyzer measurement The spectrum analyzer can measure microvolt voltage signals and can be used with a high-impedance input probe to measure the LDO output AC voltage. However, spectrum analyzer high-impedance input probes 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. 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. The THS3120 is used as the DC voltage and AC voltage superposition circuit, and the THS3120EVM is used to change it to the circuit shown in Figure 6. The OPA211 is selected to design the 100-fold amplification circuit shown in Figure 7. Figure 6 THS3120 DC voltage and AC voltage superposition circuit

  Figure 7 The power supply voltage of 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.

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

  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 as 60.4dB, which is close to 62dB in the datasheet. By changing the AC voltage frequency, the PSRR at different frequencies can also be measured. If you use an input injector and a network analyzer, you can easily measure the LDO in the design If you do not have an input injector and a network analyzer, you can select a combination of input and output listed above, then set a frequency, measure the AC voltage amplitude and the input and output voltage, and use the formula 1 to obtain the PSRR, then change the input AC signal frequency and repeat the measurement, and finally obtain the PSRR curve in the entire frequency range.