NCV8852 for in-vehicle USB power supply (Part 2)

7. Selection of output capacitor

The output capacitor ripple mainly consists of two parts, one is the ripple caused by the capacitor ESR, and the other is the ripple generated by the capacitor.



If an electrolytic capacitor is selected, it is necessary to ensure that the effective value of the output capacitor current is less than the maximum ripple current allowed by the electrolytic capacitor.

If a 22uF ceramic capacitor is selected, the ripple generated by the ESR can be ignored, and the output ripple is:

8. Selection of input capacitor Ignoring the inductor current ripple, the effective value of the input capacitor current is:

…………(25)

If an electrolytic capacitor is selected, it is necessary to ensure that the effective value of the output capacitor current is less than the maximum ripple current allowed by the electrolytic capacitor.

9. The feedback resistor is based on the formula:

…………(26)

VFB=0.8V, select RFB1=10K, then RFB2=52.5K

10. Compensation circuit setting

NCV8852 adopts peak current mode control.

 

Consider a simplified peak current model (without considering slope compensation). As shown in Figure 5:

Figure 5. Simplified peak current model RO is the equivalent load resistance.


The system has one pole and one zero. If ceramic capacitors are used, the zero caused by ESR can be ignored. The

voltage-type operational amplifier used by NCV8852 has a feedback circuit as shown in Figure 6.

Figure 6. NCV8852 output voltage error amplifier writes out the transfer function of VO to VCOMP.

…………(29)

Excluding the pole at the zero point, there is still one zero point and one pole. If CCC is not connected, there is only one zero point.

Usually, when the gain curve of the system can cross the 0db line with a slope of -1 (-20db/dec), a better phase sensitivity can be obtained. To do this, it is necessary to satisfy

：…………(30)

Fc is the crossover frequency of the system open-loop transfer function, which is usually taken as less than 1/6 of the switching frequency, FRC is the frequency of the pole of the main circuit transfer function, AP0 is the low-frequency gain of the main circuit transfer function, and AEM is the intermediate-frequency gain of the feedback circuit transfer function.



The zero point of the feedback circuit should be placed near the pole of the main circuit, so:

Among them: CC = 7.1nF is calculated, and CC = 8.2nF is selected. Because the circuit uses ceramic capacitors, there is no need to eliminate the zero point caused by ESR. CCC is not connected. If electrolytic capacitors are used, the pole of the feedback circuit needs to be placed near the ESR zero point of the main circuit. CCC can be calculated according to the following formula


Figure 7. NCV8852 5V, 2.5A Car USB Power Supply Design Example Circuit

Figure 1, Figure 7 Design Example Main Components List


Figure 8. Measured working waveform of NCV8852 circuit