LM2622: High ripples in LM2622

Hi Michael 

I think you may have a typo in "LM2622" and "LM2623". The first schematic and waveform is LM2623 and the second one is LM2622, right?

For LM2622,

• is the 370mA a continuous load or a pulse load? LM2622 can't output 370mA(not guaranteed) when working in 3.9V to 12V. Since its min current limit is 1A only. When calculating the min guaranteed maximum output current, please always use the min current limit, not the typical current limit.
• the compensation components of  C128 and R233 is not appropriate. Suggested values are 24kΩ and 2.2nF.
• could you also help measure the SW voltage, inductor current and Vin together with the Vout?
• let's further check with more information collected.

For LM2623,

• the schematic is clear enough, but the waveform picture is too small and fuzzy. Please provide a picture like the other waveform, which is clean.
• please also help measure the SW voltage, inductor current and Vin together with the Vout.
• it's better to have zoom in pictures in the part where the DC Vout is stable but the ripple is big.

Thank you.

Regards

Lei

Hi Lei, Thank you for your fast reply. 

let's first solve the LM2622 and then move on to the LM2623. 

I have changed the R233 and C180 according to your recommendation (24kOhm and 2.2nF)

this is the Scope picture I have taken for LM2622 with you request. I was not able to measure to current on the inductor. 

Yellow - Vin 

Green - SW pin (pin 5)

Brown - FB pin 

Blue - Vout 

no load - 

1.1A load - 

I also added the drop of the voltage according to the load - 

starting with No load, 500mA, 600mA, 700mA.. up to 1.1A 

Thanks, Michael 

Adding some data for LM2623 - 

Voltage degradation - 

measured under  -  No load, 500mA,600mA,700mA... 1.1A - 

why am I seeing such harsh degradation of voltage? 

picture of scope - (Yellow - Vin, green - SW, brown - FB, blue - Vout)

no load - 

1.1A load - 

Another questions -

1. I Tried to add Tantalum capacitor to the output and input instead of ceramic. should i see any different ? or ceramic should be good enough? 

2. Do you want to see the layout of the PCB ? 

Thank you, 

Michael. 

Hi Michael,

Usually we measure the inductor like this way: lift one side of the inductor, and then connect one wire between the pads on the inductor and EVM. Following is an example.  

In some of the pictures, there are more than one waveform(sun as SW voltage) shown together. You may try to zoom in and cancel zoom in immediately, then only one waveform is left and the picture will be more clean and normal.

Go back to the device of LM2622

• When working from 3.9V to 12V, the maximum Iout is around 150mA. So if the load current is 500mA or bigger, the Vout can't stay in the regulated voltage of 12V. I don't see abnormal behavior in the following picture. 

• When no load, the Vout is stable now, right?
• 
• Please don't add such a heavy load of 1.1A for LM2622. What's the maximum needed Iout? I think we need to choose other boost with bigger current limit.

For LM2623, it's similar that the maximum Iout is around 430mA when working from 3.9V to 12V. Bigger Iout will pull the Vout down.

Regards

Lei

Hi Lei

1. where i can find that information in data sheet that of LM2622 for output of 12V maximum Iout is around 150mA?

2. where i can find that information in data sheet that of LM2623 for output of 12V maximum Iout is around 430mA?

- Please guide me on Data Sheet, so I don't make a mistake in the future 

3. In my solution, i need supply 12V to continuous solenoid that require 800mA for 2 Sec. can i do some changes on IC  LM2622  or LM2623. i prefer to stay with it because I done the layout and have PCB 

4. if LM2623 and LM2622 is not Suitable for my solution (solution12V to continuous 800mA). can you suggest pin to pin soliton that can supply to out 12V/800mA ?

Thank you very much for your support !! and quick response

Michael 

Hi Michael,

You are welcome.

Sorry there is a small mistake in my previous calculation. The estimated maximum Iout when working from 3.9V to 12V is corrected below:

LM2622: 210mA.

LM2623A: 520mA.

Here is the way to estimate, using LM2622 as an example:

******************* Input *******************

vin = 3.9
vout = 12
L = 10/1000000
f = 600000
efficiency = 0.85
current_limit = 1

**********************************************

******************* Output ******************

D = 1-efficiency*vin/vout
DT = D/f
il_ripple = vin/L*DT
i_in_max = current_limit-il_ripple/2
i_out_max = i_in_max*vin*efficiency/vout

**********************************************

The input values come from the work conditions and datasheet. 

Again let's go back to your application. Since the gap between the max Iout and the needed Iout is big, and the load during is long (~2s), to make the Vout still high enough (bigger than Vin) during the 800mA load is coming, very big Cout with capacitance around 200mF is needed. Is it acceptable?

If not, let's check whether p2p solution is available tomorrow.

BTW, could you send me the detailed load (Iout) profile? Such as after the 2s duration of 800mA, how long is the time of no load or small load?

Regards

Lei

Hi, thank you for your fast reply. 

I want a temporary solution for my project until i change it to a bigger step-up. 

If I change R225 and R228 in the following schematics to Diodes - 

12VDC (1).pdf

will I be able to get 730mA (summing both currents) on the output ? 

Thank you, 

Michael

Hi 

1. what are the calculation for LM2623? i believe they are slightly different. 

2. Is it possible the change the inductance in order to increase the maximum output current ? 100uH instead of 10uH can increase my current by 50mA 

3. large capacitor is not a good solution to my project because I use solenoid. we switch the voltage every 10mS sometimes  

thank you for your replies.

Michael 

Hi Lei, Thank you for your reply. You are very helpful 

I was able to reduce my current limitation for around 500mA temporary. 

I tried to load the LM2623 with 500mA and it dropped to 10.2v.

I wish to ask few last things - 

1. Do you know how can I improve my design to work at 500mA ? Maybe change the Inductance ? 

2. Do you know of a good solution to step-up from 3.7-3.9v to 12v with 1A? 

Thank you very much for your time. 

Michael