LT8410-1 Booster Circuit Current Draw

Hi, Garibaldi,

Please post details of your actual schematic, for both cases. 

Regards

Hello Dulcevida,

Thanks for the quick answer. Please check my schematic below:

The intention is the following:

From a source that varies from 3.4 to 4.2V produce an HV_BIAS voltage that can be switchable between 53V and 105V. This is achieved by boosting the voltage of the source to either 17.6V or 34.5V (with LT8401) and then using 3 voltage doubling stages utilizing capacitor/diode doubling.

Power is fed from connector P1. Connector P3 is there for me to check the difference between connecting the output to CAP or VOUT and see the efficiency improvement. Transistor Q2 is used to switch in an 100K resistor so that selection between 53 and 105V can be done.

The circuit using the LT6106 is used for automatic overcurrent protection. The intention is that if an overcurrent event occurs, the output of LT6106 will drive the gate of Q1 high and effectively shutdown the LT8401. /SHDN is also driven by an MCU pin so that the MCU can control the part in ON/OFF.

Capacitors C22,C23,C28,C23,C27 are shown as 50V but I have also tried 100V.

Inductor L5 is MURATA part no LQH3NPN251MGRL

P6 is used to connect a trimmable load.

I am also attaching LTSpice simulation model.

Prototype circuit behaviour is the following:

  1.  When operating at 53V with NO LOAD, the current drawn by the circuit is measured as about 138uA. This is as expected per simulation and datasheet.

  1.  When operating at 105V with NO LOAD, the current drawn by the circuit is measured as 3.33mA. This is way off what is shown by simulation and datasheet.

I have tried the following to find out what is the case:

  1.  Change capacitors C22,C23,C28,C23,C27  from 50 to 100V in case of leakages

  2.  Removed D6 (which is not actually needed)

  3.  Tried connecting the output to CAP or VOUT via P3

  4.  Measured the resistance from various nodes to GND to check if there is a low impedance path (due to PCB manufacturing etc). Could not find anything low enough to justify the circuit consumption.

I would be grateful if you could provide some support on this. If you need layout files it would be ok for me to provide.

Thank you for your help.

Best regards,

Vasileios

I do not see any circuits/schematics attached to the message. The LT8401 you are discussing now I assume is a typo, and you really mean LT8410-1 as in your original message. 

The LT8410-1 has only 6mA of switch guaranteed current, about 8mA typical. I am not surprised you are seeing about 3mA current draw from your battery. I am surprised the output voltage is able to increase to 105V, as you report. 

If you can send at least the simulation you are using, I will take a look. What I suspect now is that your circuit is running at current limit, just trying to keep the output up, and the switching frequency is probably >1MHs, maybe 2MHz. If that's the case, then, the average input will be high, as you report.

Hi Dulcevida,

Yes, it is a typo. I mean LT8410-1.

I attached both the schematic and simulation files and I can see them below my post. Can you confirm that they do not show up? Can you provide a company mail to send them directly to you?

As for the part, it is running at 34.5V output which is tribbled to get to 105V. 

I understand what you are saying but we are talking about the no-load situation. The part should only be regulating the output and the current draw should be close to the quiescent for this given part for which at 34.5V output is given (in the datasheet) to be about 300uA.

Please check to see my files.

I have received it now.

Your simulation runs at about 650KHz when the output low (about 50V), and the switch current peaks at about 3mA with about 10% duty. 

When the output is high, the frequency shifts to about 1MHz, and the switch current peaks at about 8mA with >50% duty.

Seems normal to me.

These results might not be exactly as the circuit on the bench, but are consistent with what i think should be happening. As you increase the output, even at no load, the converter has to work harder to charge all the caps to the higher level. That becomes more difficult with low input voltages.

Please set your simulation for 100Vout, and increase the input to 8V. Then notice how the frequency shifts down to about 10KHz, the peak switch current drops to about 4mA. That yields a duty cycle of <1%, for a very low average current from the battery.

I think your application is working as expected.

Hello Dulcevida,

Thank you for your reply. 

I have run the simulation and can confirm what you are saying but.....what started all this is in your very last sentence. 

'That yields a duty cycle of <1%, for a very low average current from the battery.'

When I run the simulation I am measuring the average current out of the source V1. This in the case of 50V output with 3.4V input is in the order of 200uA and generally agrees with bench measurements. 

In the case of 100V output the simulation shows an average current of about 420uA coming out of the source. 

Although the switch current may peak at 3 and 8mA respectively as you mention, the average source current is much much smaller than this. 

The question then arises is the average source current that we should be looking at or the rms? The rms is shown as indeed much higher in the simulation and closer to the actual bench measurement (about 3.5mA both in simulation and bench.) However the rms when the output is 50V is simulating as 1.2mA which is way higher than in the bench measurement. 

Also all of the above are with 4uA load at 105V output and 2uA load at 53V (due to load resistor). 

When the load is removed, the average current out of the source drops to 300uA (in simulation) which is CONSISTENT with the datasheet value shown for quiescent current in regulation with no load for the part:

At 34.5V output the current is shown as about 300uA.

So my opinion is that there is still something wrong with my circuit that causes excessive current draw from the supply at 100V. 

It would be great if you could help me more on this. 

Thank you.

Also Dulcevida,

An additional note:

Take a look at the end of the datasheet of Lt8410. Essentially this is the circuit I used. According to the efficiency vs load graph at 9uA load the efficiency should be about 40% with VIN=5V and VOUT=100V.

This means that Iin=450uA.

Would there be such a deterioration in efficiency from Vin=5V to Vin=3.4V to justify an 6 fold increase in input current?

Hi Garibaldi, 

I believe the LT8410 circuit you mentioned would work for you with no problem. This part has >3 times the switch current capability.

Efficiency does degrade with lower inputs, but I think the main issue is the lT8410-1 just does not have enough juice to make it to the finish line, so it gets stuck peddling as fast as it can.

Please try the LT8410 and let me know if that solves your problem. 

Hello Dulcevida,

Thank you for your reply. 

I selected LT8410-1 in the first place because of its 8mA current limit. I am only using a very small battery and would not like to drain it more than 8mA at anytime for the booster (since there are other loads on it).

I don't think that using the LT8410 will have much different results. Response time will be better but I am not sure that consumption will be lower. The LT8410-1 does exactly what I need it to do. It has great response time and very good load and line regulation. My circuit does reach 105V from 3.4V which is also great. Also at 50V output the bench measurement matches simulation. 

The question is very simple:

the graph above shows the simulated final output voltage at 105V and the average battery draw at 308uA. The load is 4uA @105V.

My bench measurement shows current draw of about 3.5mA. 

The simulated switch voltage with no load can be seen below. The OFF time is about 830nsec:

The actual bench measured switch voltage (blue trace) with no load can be seen below:

The OFF time is more than a microsecond and switching frequency is less than a MHz. It looks to me that the part is operating in the discontinuous mode. 

All of the above seem to be very consistent with the datasheet. Hence, either my circuit/layout has some other fault or the simulation is not reliable or the datasheet has some problematic information.

Which one is true?

Hi, Garibaldi,

The only evidence I have of this application working well is shown in the datasheet circuit, with lt8410. The included datasheet efficiency curve indicates the input current should be lower than 3.5mA with no load. We have not tested this circuit with the lt8410-1, and based on what I see from your bench test results it does not work well. Your bench test waveform indicates high ON time, and close to 1MHz switching frequency. It also indicates normal switching behavior, which implies to me the layout is probably not the issue.

You indicate you want consumption to be lower, and I am saying you will get precisely that, if you copy the datasheet circuit as is.

I will gladly offer additional assistance if you try the suggested lt8410 application and you find it does not work.