Datasheet LTC3109 (Analog Devices) - 10

LTC3109
operaTion (Refer to the Block Diagram)
VOUT storage capacitor is still charging. In the event of a
PGOOD
step load on the LDO output, current can come from the A power good comparator monitors the V main V OUT voltage. OUT reservoir capacitor. The LDO requires a 2.2µF The PGOOD pin is an open-drain output with a weak pull- ceramic capacitor for stability. Larger capacitor values up (1MΩ) to the LDO voltage. Once V can be used without limitation, but will increase the time OUT has charged to within 7.5% of its programmed voltage, the PGOOD it takes for all the outputs to charge up. The LDO output output will go high. If V is current limited to 5mA minimum. OUT drops more than 9% from its programmed voltage, PGOOD will go low. The PGOOD
V
output is designed to drive a microprocessor or other
OUT
chip I/O and is not intended to drive a higher current load The main output voltage on VOUT is charged from the VAUX such as an LED. The PGOOD pin can also be pulled low in supply, and is user-programmed to one of four regulated a wire-OR configuration with other circuitry. voltages using the voltage select pins VS1 and VS2, ac- cording to Table 2. Although the logic-threshold voltage
VOUT2
for VS1 and VS2 is 0.85V typical, it is recommended that V they be tied to ground or VAUX. OUT2 is an output that can be turned on and off by the host using the VOUT2_EN pin. When enabled, VOUT2 is con-
Table 2
nected to VOUT through a 1Ω P-channel MOSFET switch.
VS2 VS1 V
This output, controlled by a host processor, can be used
OUT
GND GND 2.35V to power external circuits such as sensors and amplifiers, GND VAUX 3.3V that don’t have a low power “sleep” or shutdown capabil- VAUX GND 4.1V ity. VOUT2 can be used to power these circuits only when VAUX VAUX 5V they are needed. When the output voltage drops slightly below the regulated Minimizing the amount of decoupling capacitance on value, the charging current will be enabled as long as VAUX VOUT2 enables it to be switched on and off faster, allow- is greater than 2.5V. Once V ing shorter pulse times and therefore smaller duty cycles OUT has reached the proper value, the charging current is turned off. The resulting in applications such as a wireless sensor/transmitter. A ripple on V small VOUT2 capacitor will also minimize the energy that OUT is typically less than 20mV peak to peak . will be wasted in charging the capacitor every time VOUT2 The internal programmable resistor divider, controlled by is enabled. VS1 and VS2, sets VOUT , eliminating the need for very high value external resistors that are susceptible to noise VOUT2 has a current limiting circuit that limits the peak pickup and board leakages. current to 0.3A typical. In a typical application, a reservoir capacitor (typically a The VOUT2 enable input has a typical threshold of 1V few hundred microfarads) is connected to V with 100mV of hysteresis, making it logic compatible. If OUT . As soon as VAUX exceeds 2.5V, the V VOUT2_EN (which has an internal 5M pull-down resistor) OUT capacitor will begin to charge up to its regulated voltage. The current available is low, VOUT2 will be off. Driving VOUT2_EN high will turn to charge the capacitor will depend on the input voltage on the VOUT2 output. and transformer turns ratio, but is limited to about 15mA Note that while VOUT2_EN is high, the current limiting cir- typical. Note that for very low input voltages, this current cuitry for VOUT2 draws an extra 8µA of quiescent current may be in the range of 1µA to 1000µA. from VOUT . This added current draw has a negligible effect 3109fb 10 For more information www.linear.com/LTC3109 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 Package Description Typical Application Related Parts