In the LM5118 datasheet, Application and Implementation section, "Minimum load current (CCM operation) = 600mA", what does it mean?

yhyworld

In the LM5118 datasheet, Application and Implementation section, "Minimum load current (CCM operation) = 600mA", what does it mean? [Copy link]

See Figure 1,
LM5118 specification, 8.2.1 Design Requirements, minimum load current (CCM operation) = 600mA.

Figure 2 is Figure 19 mentioned in the design requirements.

1. What does "minimum load current (CCM operation) = 600mA" mean
here?
2. If there is no 600mA load current, can the buck-boost circuit still work properly?
3. Do all buck-boost power supplies designed with LM5118 have this minimum load current requirement?

Thank you so much!

Note:
1. CCM: Continuous Conduction Mode;
2. The attached is the LM5118 specification sheet.

Figure 1

Figure 2

appendix

lm5118.pdf (1.31 MB, downloads: 0)

2019-11-20 16:37 上传

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qwqwqw2088

DCDC continuous conduction mode minimum load current,,,,

qwqwqw2088

Switching devices work in CCM mode when the load current is large.

Usually the inductor current works in CCM continuous current mode, and the average current of the inductor is the output load current. When the load current decreases to a certain value, the converter enters the critical current mode.

maychang

1. What does "minimum load current (CCM operation) = 600mA" mean here?

This is the minimum load current required for the switching power supply to operate in the continuous current mode (CCM) under the condition of a certain inductance (probably 10uH as marked in the figure). If the load current is less than this value (the load resistance is large), it cannot operate in the continuous current mode and will enter the discontinuous current mode.

maychang

3. Do all buck-boost power supplies designed with LM5118 have a minimum load current requirement?

Switching power supplies usually have two operating modes: continuous current and discontinuous current. When the load current drops to a certain level, it will switch from continuous to discontinuous. This depends on the switching operating frequency, duty cycle, inductance, etc. When these conditions change, the minimum load current to ensure continuous current will also change.

maychang

2. If there is no 600mA load current, can the buck-boost circuit still work properly?

I don't know about this, I haven't looked at the chip datasheet carefully.

The transfer functions of the two working conditions are different when working in the continuous current mode and the discontinuous current mode. In one working condition, the negative feedback is stable, but it may become unstable and may oscillate when it is transferred to another working condition due to the change of load current. Of course, the feedback can also be designed to be stable in both working conditions, but that often sacrifices the rapid response.

PowerAnts

The minimum CCM current, also known as the critical mode current, is used to calculate the inductance. Almost all controllers are not stupid enough to specify a minimum load current.

The statement on the third post is wrong. If it is a BUCK, no matter what mode it is, the average inductor current is equal to the load current. For BUCK and BUCK-BOOST, the average inductor current must be greater than the load current.

PowerAnts

The original poster may not understand the inductor ripple factor of the BUCK. TI recommends 0.4 in most cases. The 600mA in the screenshot is the 3A rated current in the datasheet. The inductor peak current in critical mode = 0.4*3=1.2A, so the corresponding I(load)=1.2/2=0.6(A)

yhyworld

qwqwqw2088 Published on 2019-11-20 17:31 Switching devices operate in CCM mode when the load current is large. Usually, the current of the inductor operates in CCM continuous current mode. The average current of the inductor...

Thank you, moderator! Got it!

yhyworld

maychang posted on 2019-11-20 17:50 2. If there is no 600mA load current, can the buck-boost circuit still work properly? I don't know this because I haven't read the chip datasheet carefully. ...

Thank you Teacher Maychang for your patient guidance!

From XX forum till now, I can always see you............

yhyworld

PowerAnts posted on 2019-11-20 22:35 The OP may not understand the inductor ripple factor of BUCK. TI recommends 0.4 in most cases, and the 600mA in the screenshot is the 3A rated current in the datasheet...

Thank you PowerAnts senior for your patient guidance!