Datasheet LTC3108-1 (Analog Devices) - 10

LTC3108-1
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 go high.
VSTORE
If VOUT drops more than 9% from its regulated voltage, The VSTORE output can be used to charge a large storage PGD will go low. The PGD output is designed to drive a capacitor or rechargeable battery after V microprocessor or other chip I/O and is not intended to OUT has reached regulation. Once V drive a higher current load such as an LED. Pulling PGD OUT has reached regulation, the VSTORE output will be allowed to charge up to the VAUX voltage. up externally to a voltage greater than VLDO will cause a The storage element on VSTORE can be used to power small current to be sourced into VLDO. PGD can be pulled the system in the event that the input source is lost, or low in a wire-OR configuration with other circuitry. is unable to provide the current demanded by the VOUT, V
V
OUT2 and LDO outputs. If VAUX drops below VSTORE,
OUT2
the LTC3108-1 will automatically draw current from the VOUT2 is an output that can be turned on and off by the storage element. Note that it may take a long time to charge host, using the VOUT2_EN pin. When enabled, VOUT2 is a large capacitor, depending on the input energy available connected to VOUT through a 1.3Ω P-channel MOSFET and the loading on VOUT and VLDO. switch. This output, controlled by a host processor, can Since the maximum current from VSTORE is limited to a be used to power external circuits such as sensors and few milliamps, it can safely be used to trickle-charge NiCd amplifiers, that do not have a low power sleep or shutdown or NiMH rechargeable batteries for energy storage when capability. VOUT2 can be used to power these circuits only the input voltage is lost. Note that the VSTORE capacitor when they are needed. cannot supply large pulse currents to VOUT. Any pulse load Minimizing the amount of decoupling capacitance on on VOUT must be handled by the VOUT capacitor. VOUT2 will allow it to be switched on and off faster, allowing shorter burst times and, therefore, smaller duty cycles in
Short-Circuit Protection
pulsed applications such as a wireless sensor/transmit- All outputs of the LTC3108-1 are current limited to protect ter. A small VOUT2 capacitor will also minimize the energy against short-circuits to ground. that will be wasted in charging the capacitor every time VOUT2 is enabled.
Output Voltage Sequencing
VOUT2 has a soft-start time of about 5µs to limit capacitor A timing diagram showing the typical charging and charging current and minimize glitching of the main output voltage sequencing of the outputs is shown in Figure 1. when VOUT2 is enabled. It also has a current limiting circuit Note: time not to scale. that limits the peak current to 0.3A typical. 31081fb 10 For more information www.linear.com/LTC3108-1 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Typical Application Related Parts