Datasheet LTC3108 (Analog Devices) - 10

link to page 11 LTC3108
OPERATION
In a typical application, a storage capacitor (typically a few The VOUT2 enable input has a typical threshold of 1V hundred microfarads) is connected to VOUT. As soon as with 100mV of hysteresis, making it logic-compatible. If VAUX exceeds 2.5V, the VOUT capacitor will be allowed to VOUT2_EN (which has an internal pull-down resistor) is charge up to its regulated voltage. The current available low, VOUT2 will be off. Driving VOUT2_EN high will turn on to charge the capacitor will depend on the input voltage the VOUT2 output. and transformer turns ratio, but is limited to about 4.5mA Note that while V typical. OUT2_EN is high, the current limiting cir- cuitry for VOUT2 draws an extra 8µA of quiescent current
PGOOD
from VOUT. This added current draw has a negligible effect on the application and capacitor sizing, since the load on A power good comparator monitors the VOUT voltage. the VOUT2 output, when enabled, is likely to be orders of The PGD pin is an open-drain output with a weak pull-up magnitude higher than 8µA. (1MΩ) to the LDO voltage. Once VOUT has charged to within 7.5% of its regulated voltage, the PGD output will
VSTORE
go high. If VOUT drops more than 9% from its regulated The VSTORE output can be used to charge a large stor- voltage, PGD will go low. The PGD output is designed age capacitor or rechargeable battery after V to drive a microprocessor or other chip I/O and is not OUT has reached regulation. Once V intended to drive a higher current load such as an LED. OUT has reached regulation, the VSTORE output will be allowed to charge up to the VAUX Pulling PGD up externally to a voltage greater than VLDO voltage. The storage element on VSTORE can be used will cause a small current to be sourced into VLDO. PGD to power the system in the event that the input source can be pulled low in a wire-OR configuration with other is lost, or is unable to provide the current demanded by circuitry. the VOUT, VOUT2 and LDO outputs. If VAUX drops below VSTORE, the LTC3108 wil automatical y draw current
VOUT2
from the storage element. Note that it may take a long VOUT2 is an output that can be turned on and off by the time to charge a large capacitor, depending on the input host, using the VOUT2_EN pin. When enabled, VOUT2 is energy available and the loading on VOUT and VLDO. connected to VOUT through a 1.3Ω P-channel MOSFET Since the maximum current from VSTORE is limited to a switch. This output, controlled by a host processor, can few milliamps, it can safely be used to trickle-charge NiCd be used to power external circuits such as sensors and or NiMH rechargeable batteries for energy storage when amplifiers, that do not have a low power sleep or shut- the input voltage is lost. Note that the VSTORE capacitor down capability. VOUT2 can be used to power these cir- cannot supply large pulse currents to V cuits only when they are needed. OUT. Any pulse load on VOUT must be handled by the VOUT capacitor. Minimizing the amount of decoupling capacitance on V
Short-Circuit Protection
OUT2 will allow it to be switched on and off faster, allow- ing shorter burst times and, therefore, smaller duty cycles All outputs of the LTC3108 are current limited to protect in pulsed applications such as a wireless sensor/transmit- against short-circuits to ground. ter. A small VOUT2 capacitor will also minimize the energy that will be wasted in charging the capacitor every time
Output Voltage Sequencing
VOUT2 is enabled. A timing diagram showing the typical charging and VOUT2 has a soft-start time of about 5µs to limit capacitor voltage sequencing of the outputs is shown in Figure 1. charging current and minimize glitching of the main out- Note: time not to scale. put when VOUT2 is enabled. It also has a current limiting circuit that limits the peak current to 0.3A typical. Rev. D 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts