The LTC3105 has the capability to start with voltages as low as 250mV. During start-up the AUX output initially is charged with the synchronous rectifiers disabled. Once VAUX has reached approximately 1.4V, the converter leaves start-up mode and enters normal operation. Maximum power point control is not enabled during start-up; however, the currents are internally limited to sufficiently low levels to allow start-up from weak input sources. While the converter is in start-up mode, the internal switch between AUX and VOUT remains disabled and the LDO is disabled. Refer to Figure 3
for an example of a typical start-up sequence.
When either VIN or VAUX is greater than 1.4V, the converter will enter normal operation. The converter continues charging the AUX output until the LDO output enters regulation. Once the LDO output is in regulation, the converter begins charging the VOUT pin. VAUX is maintained at a level sufficient to ensure the LDO remains in regulation. If VAUX becomes higher than required to maintain LDO regulation, charge is transferred from the AUX output to the VOUT output. If VAUX falls too low, current is redirected to the AUX output instead of being used to charge the VOUT output. Once VOUT rises above VAUX, an internal switch is enabled to connect the two outputs together.
If VIN is greater than the voltage on the driven output (VOUT or VAUX) or the driven output is less than 1.2V, the synchronous rectifiers are disabled and operate in critical conduction mode, enabling regulation even when VIN>VOUT.
When the output voltage is greater than the input voltage and greater than 1.2V, the synchronous rectifier is enabled. In this mode, the N-channel MOSFET between SW and GND is enabled until the inductor current reaches the peak current limit. Once current limit is reached, the N-channel MOSFET turns off and the P-channel MOSFET between SW and the driven output is enabled. This switch remains on until the inductor current drops below the valley current limit and the cycle is repeated. When VOUT reaches the regulation point, the N- and P-channel MOSFETs connected to the SW pin are disabled and the converter enters sleep.
Figure 3: Typical LTC3105 start-up sequence
In order to power microcontrollers and external sensors an integrated LDO provides a regulated 6mA rail. The LDO is powered from the AUX output allowing the LDO to attain regulation while the main output is still charging. The LDO output voltage can be either a fixed 2.2V or adjusted via resistor divider.
The integrated maximum power point control circuit allows the user to set the optimal input voltage operating point for a given power source - see Figure 4
. The MPPC circuit dynamically regulates the average inductor current to prevent the input voltage from dropping below the MPPC threshold. When VIN is greater than the MPPC voltage, the inductor current is increased until VIN is pulled down to the MPPC set point. If VIN is less than the MPPC voltage, the inductor current is reduced until VIN rises to the MPPC set point.
Figure 4: Typical maximum power point control point for a single photovoltaic cell