# LTC3105 Energy Harvester taking more current from input without any output load

Hello sir,

We are using LTC3105 Energy Harvester in one of project.

We are giving input voltage to LTC3105 using Variable power supply

We designed LTC3105 ouput to 4.3V

I given input from 55 mV to 450 mV

We tested this LTC 3105 wtihout any output load

I observed following reading from this test

Input Voltage ( mV )         Input current ( mA )          Output Voltage ( Volts )

55 mV                                   0                                          0.019 V

100 mV                                 0                                          0.01  V

150 mV                                 130                                      1.39 V

250 mV                                 230                                      1.572 V

300 mV                                290                                        1.668 V

350 mV                                 390                                       2 V

415 mV                                 0                                          4.3 V

450 mV                                 0                                          4.35 V

Without any output load LTC3105 consuming current up to 390 mA at output voltage = 2V

when LTC3105 achieves the programmed output voltage the current consumption of the regulator is 0A.

May I know the reason for the above scenario.

Thank you

Regards,

Lakshmanan V

PDF

• Hi,

Minimum start-up voltage for LTC3105 is 0.25V . In page 9 of the datasheet you can see the start-up sequence of the converter.  First VAUX and then VLDO needs to be charged up before Vout to start up. After LDO is in regulation, AUX starts to charge VOUT till it is in regulation.

During this process the converter has to step up from very low input voltage  to higher output voltage. Since the boost ratio is too high, the input current increases to supply power to step up the voltage till it hits the current limit.  So what you have observed in the test is the general startup sequence of the converter.

If the initial inductor current overshoot to peak current limit is not desirable in your application, you can use the mppc function. For example if the startup voltage in your application is 0.4V then mppc voltage can be set to 0.35V or lower. This will delay the startup by  limiting the inductor current for input voltage less than mppc but you will not see the overshoot in the inductor current.

Also if the startup input voltage is 0.25V and your input source has lower current capability it is recommended to use high input capacitance (atleast 470uF). As mentioned above the input capacitance should be high enough such that it should be able to charge up both AUX and LDO during the startup which will further charge Vout to regulation else it will never come out of the startup mode.

Thanks