problem with lm-2577t-adj

[yoosefheidari]

hi guys
i use a lm2577t-adj to boost voltage of an one cell li-ion battery(3.9volts)to 6.4volts.i want drive 4 1watt led's.each two of led's are series and then i parallel that two strings.
when i connect led's to circuit voltage drop to 5.5 volts.
what is problem?by set output voltage to higher voltage is 5.5volts
in fact i want convert 3.9volts to 6.4volts with 0.7amps of current.(pout=6.4*.7 from a 3.9 volt battery and input curarent is about 1.3amps)
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another question is how can i calculate suitable amount af inductor?

 

[schmitt trigger]

Do the LED loads have a limiting resistor?
An LED, which is a diode, will clamp any voltage to its forward voltage Vf.

When one uses a converter to power up LED, one has to regulate Current instead of voltage. That means a current sampling resistor in series with the LED that will generate a voltage directly proportional to the current.

 

[yoosefheidari]

ok
i series a 28 ohm resistor with led's and output voltage is 5.5
why voltage can't go higher?
we should regulate current when we have at least 6.4volts in output.is it right?

 

[schmitt trigger]

On page 15 of the datasheet, there is the following STEP-UP REGULATOR DESIGN FORMULAS, the formulas are in image mode, so I cannot copy and past them here, please download the datasheet and follow the next steps:

Calculate Duty Cycle D using you actual circuit parameters and minimum input voltage.

Calculate Inductor Current Ripple delta-Iind

Calculate the Peak Switch Current Isw(pk)

Will these value be lower than the rated maximum switch current = 3 amps?

 

[yoosefheidari]

i use these values
Vin=3.9
Vout=6.4
Vsat=.5
L=100uH
Iload (max)=0.7A
and result:
D=0.39
DELTA-IND=0.255
Isw(pk)=1.27
so what should i do?

 

[schmitt trigger]

Ok, this means that based on calculated values, you are not overloading it.

Do you have access to an oscilloscope? Checking waveforms would be very useful

Also...to rule out that the LEDs are not clamping down the supply, do you have, or can get a 10 ohm, 5 watt resistor that you can use as a temporary load?

 

[yoosefheidari]

i haven't an oscilloscope
i buy a resistor tomarrow and try it.
but a question:
when i touch pins of ic light of led's is going higher.why?

 

[schmitt trigger]

That is why you need a scope, there may be noise or instability.

If you are going to buy the resistor, get yourself better a pair of 20 ohm resistors. Then by either series or parallel arrangements, you can get 40, 20 or 10 ohm, to test the circuit at different loads.

 

[yoosefheidari]

can a toroidal inductor solve the problem?or change capacitor with a low esr capacitor for output line?
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is 100uh inductor a good selection for 6.4volts/.7amps output?

 

[schmitt trigger]

Select the inductor according to the design formulas found in the datasheet.

What I've learned to do, is to create an Excell spreadsheet with all the design formulas. Once that it is done, you can do "what if" scenarios, changing different values, voltages and loads, and see the performance and determine if you have not exceed any parameters.

 

[Audioguru]

A Lithium battery is 4.2V when fully charged. Yours is only 3.9V so maybe it is not charged and its voltage drops lower when it has a load. Maybe the battery cannot supply 1.4A.

The datasheet for the LM2577ADJ shows spec's and circuits with a 5V input, not with a 3.9V input. Maybe the battery voltage drops lower. Maybe the performance of the IC is awful with an input voltage lower than 5V.

 

[yoosefheidari]

ok.
how can i fix output current?

 

[Audioguru]

how can i fix output current?

What is the mAh rating of the battery and why is it not fully charged?

Don't you understand that the LM2577 is not spec'd for an input less than 5V? It works well with an input of 5V or more.

 

[yoosefheidari]

What is the mAh rating of the battery and why is it not fully charged?

Don't you understand that the LM2577 is not spec'd for an input less than 5V? It works well with an input of 5V or more.

it's 2600mahand and is fully charged.
butin datasheet has been wrote input range is from 3.5 to 40 volt.why not work under 5 volts?

 

[Audioguru]

it's 2600mAh and and is fully charged.

No.
Some Lithium batteries sold on e-bay are absolute GARBAGE. One was rated at 3000mAh but reviews showed only 170mAh.
An older battery has less mAh than a new battery. A battery that is not fully charged has less maximum output current than a fully charged battery.

A rechargeable Lithium cell is 4.2V when fully charged (read all about it at Battery University) so yours is not fully charged.

but in datasheet has been wrote input range is from 3.5 to 40 volt.why not work under 5 volts?

It shows specifications only when the input is 5V or more. It works with an input of 3.5V but so poorly that its specifications are not shown.

Here is a graph of tests on Li-Po batteries for electric model airplanes. The Li-Po batteries are fully charged to 4.2V then loaded with 2A (12.5 times more than rated). Most of the batteries perform well but the cheap Turnigy Chinese batteries are awful:

 

[yoosefheidari]

i'm sure my battery is 2600mah and model of battery is c18650cc (search google for that) and voltage of fully charge is 4.2 volts.

 

[Audioguru]

Maybe your battery is poor quality and its "2600mAh" is actually much lower.
Maybe the battery is old.
Maybe the battery has a capacity of 2600mAh when its current is low like 100mA. Then its voltage will drop when it tries to supply 1.4A to your LM2577 IC.

The number 18650 means that its length is 186mm and its diameter is 50mm.
Do you have a proper charger for it? The charger should limit the current and detect when the voltage reaches 4.20V. Then the charger must keep the voltage at 4.20V while it senses the charging current. When the charging current drops to a low amount then the charger should turn off.

Do you have a proper high current holder for the battery cell?

 

[yoosefheidari]

in another circuit this battery give about 1.2 amps to that circuit.so battery can give about 1.4amps to lm2577 circuit.
my charger has good quality and turn off when voltage of battery be 4.2volts.
you search this "c18650cc"in google to see my battery pictures and specifications of battery.
i fact you say lm2577 can convert about 3.7-4.2volts/1.4amps to 6.4volts/.7amps ?