LM3475: Charger using lm3475

Onkar Bhakare

Part Number: LM3475

Hi,

I am using lm3475 for the battery charger circuit.V ref value for the hysteric comparator is 0.8V. but in my case voltage at feedback pin is 1.1V still IC does not try to maintain it at 0.8V. So output is coming more than expected.

Also sum of voltage drop across the feedback resistor is greater than output voltage .I don't understand why this is happening

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

Onkar,

Can you post your schematics? Do you have any switching at the SW node? If you can post some waveforms of your circuit operation, that would be helpful as well.

0 Onkar Bhakare over 7 years ago in reply to JohnTucker

Intellectual 550 points

Hi John,

PFA of schematic and waveforms.

<Do you have any switching at SW node ?

Yes there is switching at sw node

charger sckm.pdf charger sckm.pdf

0 JohnTucker over 7 years ago in reply to Onkar Bhakare

TI__Guru**** 191445 points

Onkar,

Your schematic does not have any values. I will need them. Also the part number of your output capacitor. LM3475 is a hysteretic controller, so your output voltage is going to be higher than you expect as the 0.8 V should be the valley of the ripple waveform and the average dc voltage will be off set by ( Vhyst * Vout * Vref) / 2. Higher Vout will have more ripple and more offset. You did not state what your input voltage, output voltage and output current conditions are. See sections 8.2.2.1 and 8.2.2.2 of the datasheet. Is the first figure the output voltage? Can you offset it so I can see the GND reference?

0 Onkar Bhakare over 7 years ago in reply to JohnTucker

Intellectual 550 points

L=15uH

Cin=47uF

Vout=3*47uF

Cff=1nF

These are the values of components

0 Onkar Bhakare over 7 years ago in reply to Onkar Bhakare

Intellectual 550 points

Vout is 4.2V
Vin varied from 4.2V to 7V

0 JohnTucker over 7 years ago in reply to Onkar Bhakare

TI__Guru**** 191445 points

Can you tell me the FB resistor values? Are the output capacitors electrolytic or ceramic? If electrolytic, what is the ESR? Let me know.

0 Onkar Bhakare over 7 years ago in reply to JohnTucker

Intellectual 550 points

Rb1=51k
Rb2=12k
Out capacitors are ceramic

0 Mathew Jacob1 over 7 years ago in reply to Onkar Bhakare

TI__Expert 5910 points

I do not see Rb1 and Rb2 on your schematic. Would recommend trying the basic controller design first and then add on the charging circuit. Please check Vin=7V and Vo=4.2V.

Regards,
Mathew

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

R2 and R3 are the feedback resistor in my schematic

0 Mathew Jacob1 over 7 years ago in reply to Onkar Bhakare

TI__Expert 5910 points

Hi,

Were you able to verify operation without the charger scheme connected?

Regards,

Mathew

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

I able to understand operation of converter.but didn't understand why the drop across FB resistor is more than output voltage

It should be equal to output voltage, isn't it?

0 Mathew Jacob1 over 7 years ago in reply to Onkar Bhakare

TI__Expert 5910 points

The drop across FB resistors top and bottom should be the same as the output voltage. If the output voltage is not regulated for some reason then the voltage would be higher than your calculated output voltage.

Regards,
Mathew

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

Yes I able to understand operation of the converter with or without charger scheme but I didn't understand the drop across FB resistor is greater than output voltage.it should be equal to output voltage,shouldn't it?

0 Onkar Bhakare over 7 years ago in reply to Onkar Bhakare

Intellectual 550 points

Sorry this send by mistake

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

But their is difference of 1V even if ripple is not so much(ripple nearly 100mV at output)

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

Can you explain me why drop across FB resistor is not same in case of unregulated output

0 Mathew Jacob1 over 7 years ago in reply to Onkar Bhakare

TI__Expert 5910 points

FB pin bottom resistor voltage drop should be equal to the reference voltage. If you measure across the top f/b divider resistor the voltage would be the output voltage minus the reference voltage.

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

This means output voltage =reference + voltage drop across 1st fb resistor but this is not happening

0 Onkar Bhakare over 7 years ago in reply to Mathew Jacob1

Intellectual 550 points

Please see my previous mail in which I mentioned that difference between output voltage and drop across fb resistors is 1V even if output ripple voltage is 100mV. Please explain in detail

0 JohnTucker over 7 years ago in reply to Onkar Bhakare

TI__Guru**** 191445 points

51k and 12k should set your output voltage to 4.2 V. At lower input voltages near 4.2 V, you will not be able to regulate as there is not sufficient headroom. Please raise the input voltage to your maximum (7 V?). Capture Vout and the SW node voltage waveform. Please ensure that the vertical scales are shown so that we can observe the full amplitude such as 1 V/div. Please ensure that the vertical ground reference is placed on a makor vertical grid line so it is easy to read the amplitude.

Thanks,

0 JohnTucker over 7 years ago in reply to JohnTucker

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Any update or additional information? If not I will close this thread. Please let us know.

0 Onkar Bhakare over 7 years ago in reply to JohnTucker

Intellectual 550 points

Sorry Currently I am not working on this circuitry so no updates

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LM3475: Charger using lm3475