Datasheet LTC3108-1 (Analog Devices) - 9

LTC3108-1
operaTion Oscillator Synchronous Rectifiers
The LTC3108-1 utilizes a MOSFET switch to form a resonant Once VAUX exceeds 2V, synchronous rectifiers in parallel step-up oscillator using an external step-up transformer with each of the internal diodes take over the job of rectify- and a small coupling capacitor. This allows it to boost input ing the input voltage, improving efficiency. voltages as low as 20mV high enough to provide multiple regulated output voltages for powering other circuits. The
Low Dropout Linear Regulator (LDO)
frequency of oscillation is determined by the inductance of The LTC3108-1 includes a low current LDO to provide a the transformer secondary winding and is typically in the regulated 2.2V output for powering low power processors range of 10kHz to 100kHz. For input voltages as low as or other low power ICs. The LDO is powered by the higher 20mV, a primary-secondary turns ratio of about 1:100 is of VAUX or VOUT. This enables it to become active as soon recommended. For higher input voltages, this ratio can be as VAUX has charged to 2.3V, while the VOUT storage lower. See the Applications Information section for more capacitor is still charging. In the event of a step load on information on selecting the transformer. the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT. The LDO requires
Charge Pump and Rectifier
a 2.2µF ceramic capacitor for stability. Larger capacitor The AC voltage produced on the secondary winding of values can be used without limitation, but will increase the transformer is boosted and rectified using an external the time it takes for all the outputs to charge up. The LDO charge pump capacitor (from the secondary winding to output is current limited to 4mA minimum. pin C1) and the rectifiers internal to the LTC3108-1. The rectifier circuit feeds current into the VAUX pin, provid-
VOUT
ing charge to the external VAUX capacitor and the other The main output voltage on VOUT is charged from the VAUX outputs. supply, and is user programmed to one of four regulated voltages using the voltage select pins VS1 and VS2, ac-
VAUX
cording to Table 2. Although the logic threshold voltage The active circuits within the LTC3108-1 are powered from for VS1 and VS2 is 0.85V typical, it is recommended that VAUX, which should be bypassed with a 1µF capacitor. they be tied to ground or VAUX. Larger capacitor values are recommended when using
Table 2. Regulated Voltage Using Pins VS1 and VS2
turns ratios of 1:50 or 1:20 (refer to the Typical Applica-
VS2 VS1 VOUT
tion examples). Once VAUX exceeds 2.5V, the main VOUT GND GND 2.5V is allowed to start charging. GND VAUX 3V An internal shunt regulator limits the maximum voltage VAUX GND 3.7V on VAUX to 5.25V typical. It shunts to GND any excess VAUX VAUX 4.5V current into VAUX when there is no load on the converter When the output voltage drops slightly below the regulated or the input source is generating more power than is value, the charging current will be enabled as long as VAUX required by the load. is greater than 2.5V. Once VOUT has reached the proper value, the charging current is turned off.
Voltage Reference
The internal programmable resistor divider sets VOUT, The LTC3108-1 includes a precision, micropower refer- eliminating the need for very high value external resistors ence, for accurate regulated output voltages. This reference that are susceptible to board leakage. becomes active as soon as VAUX exceeds 2V. 31081fb For more information www.linear.com/LTC3108-1 9 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