LM2578 buck boost converter

Hi Archana...

I'm sorry, but it's not clear to me what you need...
Do you want to run your buck boost in CCM or in DCM?
Have you any specification you can post?

Bye

Paolo

Hi Archana,

Can you provide your specs?

what value inductors did you calculate from the equation and example?

Regards,

Ismail

thanks for the response.

please see the datasheet of lm2578A. on pg 16 , lm2578A is configured in buck-boost mode and for calculation of L1 ,

the formula given is L1>=(V_in-V_sat-V_sat1)(t_on/i _p)

for my values of vin (3 to 24V)

vsat =0.7(datasheet)

vsat1=1 (d44 transistor)

fosc=50khz

Vo=5

Vd=0.6 (schottky diode)

Io=750 mA

i m getin a value as 52.6458uH when vin=24

and L = 2.47 uH when vin= 3V

please tell me why is inductor value ending up so low.

On cross referencing with other general formula for buck boost , value comes near253uH and 66.66uH

Hi Archana.

I don't use the DS formula, but with your specs it seems that required inductance for your circuit it is much more close to 2uH rather than 66uH.

My results is more or less something like 6-7uH.

The reason is quite simple!

This is the required inductance to run your circuit in DCM.

You can also select greater inductance values, but this means that you will run your circuit in CCM and this is a situation dangerour due to the fact that you have to compensate a RHP zero.

Please, let me know which is your selection in terms of DCM or CCM and I will give you more information.

Bye

Paolo

(The LM2578A may be operated in either the continuous or

the discontinuous conduction mode. The following applications

(except for the Buck-Boost Regulator) are designed for

continuous conduction operation. That is, the inductor current

is not allowed to fall to zero. This mode of operation has

higher efficiency and lower EMI characteristics than the discontinuous

mode.) specified in datsheet

i need a CCM mode

Hi Archana.

If you need to run in CCM it is sufficient to select an inductance higher than:

L>[Vout*(1-D)^2]/(2*Iout*fsw)

The higher the inductance value, the lower the ripple current...

Be careful with RHP when you close your control loop!

Bye

Paolo

Thank you paolo but i really dont understand the constraint with RHS. I can even use a 25118 LM which is a wide buck boost controller?

Thanks in advance

Hi Archana.

When you run a buck boost in CCM you have a Right Half Plane zero in your closed loop transfer function, both if you run a voltage mode than a current mode control, a RHP not present if you run your circuit in DCM.

Simply, you have to put your crossover frequency at least oen decade lower than your RHP zero frequency, or you risk to have strong instability.

Your circuit will be slower...

That's the reason for which I suggested you to run in DCM.

Bye

Paolo

ehey paolo please tell me the formula for DCM mode as well.