Datasheet LTC3105 (Analog Devices) - 8

LTC3105
OPERATION Introduction
start-up mode. Once in normal operation, the SHDN pin The LTC3105 is a unique, high performance, synchronous may be controlled using an open-drain or open-collector boost converter that incorporates maximum power point pull-down. Other external loads on this pin should be control, 250mV start-up capability and an integrated LDO avoided, as they may result in the part failing to reach regulator. This part operates over a very wide range of input regulation. In shutdown, the internal switch connecting AUX and V voltages from 225mV to 5V. Its Burst Mode architecture OUT is enabled. and low 24µA quiescent current optimize efficiency in low When the SHDN pin is released, the LTC3105 is enabled power applications. and begins switching after a short delay. When either VIN or V An integrated maximum power point controller allows for AUX is above 1.4V, this delay will typically range be- tween 20µs and 100µs. Refer to the Typical Performance operation directly from high impedance sources such as Characteristics section for more details. photovoltaic cells by preventing the input power source voltage from collapsing below the user programmable
Start-Up Mode Operation
MPPC threshold. Peak current limits are automatically adjusted with proprietary techniques to maintain operation The LTC3105 provides the capability to start with voltages at levels that maximize power extraction from the source. as low as 250mV. During start-up the AUX output initially is charged with the synchronous rectifiers disabled. Once The 250mV start-up voltage and 225mV minimum VAUX has reached approximately 1.4V, the converter leaves operating voltage enable direct operation from a single start-up mode and enters normal operation. Maximum photovoltaic cell and other very low voltage, high series power point control is not enabled during start-up, however, impedance power sources such as TEGs and fuel cells. the currents are internally limited to sufficiently low levels Synchronous rectification provides high efficiency opera- to allow start-up from weak input sources. tion while eliminating the need for external Schottky diodes. While the converter is in start-up mode, the internal switch The LTC3105 provides output disconnect which prevents between AUX and VOUT remains disabled and the LDO large inrush currents during start-up. This is particularly is disabled. Refer to Figure 1 for an example of a typical important for high internal resistance power sources like start-up sequence. photovoltaic cells and thermoelectric generators which The LTC3105 is optimized for use with high impedance can become overloaded if inrush current is not limited power sources such as photovoltaic cells. For operation during start-up of the power converter. In addition, output from very low impedance, low input voltage sources, it may disconnect isolates VOUT from VIN while in shutdown. be necessary to add several hundred milliohms of series input resistance to allow for proper low voltage start-up.
VIN > VOUT Operation
The LTC3105 includes the ability to seamlessly maintain
Normal Operation
regulation if VIN becomes equal to or greater than VOUT. When either VIN or VAUX is greater than 1.4V typical, the With VIN greater than or equal to VOUT, the synchro- converter will enter normal operation. nous rectifiers are disabled which may result in reduced The converter continues charging the AUX output until efficiency. the LDO output enters regulation. Once the LDO output
Shutdown Control
is in regulation, the converter begins charging the VOUT pin. VAUX is maintained at a level sufficient to ensure the The SHDN pin is an active low input that places the IC LDO remains in regulation. If VAUX becomes higher than into low current shutdown mode. This pin incorporates an required to maintain LDO regulation, charge is transferred internal 2MΩ pull-up resistor which enables the converter from the AUX output to the VOUT output. If VAUX falls too if the SHDN pin is not controlled by an external circuit. The low, current is redirected to the AUX output instead of SHDN pin should be allowed to float while the part is in being used to charge the VOUT output. Once VOUT rises 3105fb 8 For more information www.linear.com/LTC3105 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts