power harvesting using ltc3108

[arezu]

hello everybody
I want to use ltc3108 for boosted and power harvesting in medical and implanted devices.
how this converter works?
what is role of 220uF at input?

 

[KlausST]

Hi,

how this converter works?

LT´s datasheets are very clear in this.
There is a raw "DESCRIPTION" chapter.
and a more detailed "OPERATION" chapter.
and it has an "APPLICATION INFORMATION" chapter.

--> Read the datasheet.

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

what is role of 220uF at input?

Datasheet says:
Note that a low ESR bulk decoupling capacitor will usually
be required across the input source to prevent large voltage
droop and ripple caused by the source’s ESR and the peak
primary switching current (which can reach hundreds of
milliamps). The time constant of the filter capacitor and
the ESR of the voltage source should be much longer than
the period of the resonant switching frequency.

--> it is just a storeage device for the input energy.


Klaus

 

[arezu]

input impedance of LTC3108

Hi everybody
How can I calculate input impedance of LTC3108 ?
thanks

 

[KlausST]

Re: input impedance of LTC3108

Hi,

Input impedance = U_in / I_in.

It varies with input voltage, output current and efficiency.

You may calculate it for a fix situation only. Fix input voltage, fix output voltage, fix output current.

Klaus

 

[arezu]

Dear Klaus
I dont understand role of 220uf at input

 

[KlausST]

Hi,

Imagine a high impedance source...a small solar panel for example.

With low light level it maybe can source 1mA with max5V.
Without capacitor it can only feed 1mA to the inductor, and only fir the time when the switch is ON.
When the switch is OFF, then the energy is lost.
During OFF time the energy stored in the inductance is transferred to the output side.
The stored energy is: W = 0.5 x I x I x L...but remember the current is only 1mA, therefore only a little bit if energy.

Now imagine with capacitor. All the current from the solar cell is fed into the capacitor. All the time.
The voltage of the capacitor rises.
Then the inductance is switched ON and a large current (maybe 100mA) can be drawn from the capacitor and fed into the inductance.
During this (short) time the current from the solar cell is still fed into the capacitor.
When the switch is OFF, then the energy is fed to the output...
Because the stored energy is much more than before the switching frequency is low...increasing the efficiency..

Klaus