Datasheet LTC3108 (Analog Devices) - 9

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OPERATION Oscillator Synchronous Rectifiers
The LTC3108 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 ing the input voltage, improving efficiency. input voltages as low as 20mV high enough to provide multiple regulated output voltages for powering other cir-
Low Dropout Linear Regulator (LDO)
cuits. The frequency of oscillation is determined by the The LTC3108 includes a low current LDO to provide a inductance of the transformer secondary winding and is regulated 2.2V output for powering low power processors typically in the range of 10kHz to 100kHz. For input volt- or other low power ICs. The LDO is powered by the higher ages as low as 20mV, a primary-secondary turns ratio of of VAUX or VOUT. This enables it to become active as soon about 1:100 is recommended. For higher input voltages, as VAUX has charged to 2.3V, while the VOUT storage this ratio can be lower. See the Applications Information capacitor is still charging. In the event of a step load on section for more 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. 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 outputs. VAUX supply, and is user programmed to one of four regulated voltages using the voltage select pins VS1 and
VAUX
VS2, according to Table 2. Although the logic threshold The active circuits within the LTC3108 are powered from voltage for VS1 and VS2 is 0.85V typical, it is recom- VAUX, which should be bypassed with a 1µF capacitor. mended that 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 Application
VS2 VS1 VOUT
examples). Once VAUX exceeds 2.5V, the main VOUT is GND GND 2.35V allowed to start charging. GND VAUX 3.3V An internal shunt regulator limits the maximum voltage VAUX GND 4.1V on VAUX to 5.25V typical. It shunts to GND any excess VAUX VAUX 5V current into VAUX when there is no load on the converter or the input source is generating more power than is When the output voltage drops slightly below the regu- required by the load. lated value, the charging current will be enabled as long as VAUX is greater than 2.5V. Once VOUT has reached the
Voltage Reference
proper value, the charging current is turned off. The LTC3108 includes a precision, micropower reference, The internal programmable resistor divider sets VOUT, for accurate regulated output voltages. This reference eliminating the need for very high value external resis- becomes active as soon as VAUX exceeds 2V. tors that are susceptible to board leakage. Rev. D For more information www.analog.com 9 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