Datasheet LTC3418 (Analog Devices) - 9

LTC3418
operaTion Dropout Operation Short-Circuit Protection
When the input supply voltage decreases toward the output When the output is shorted to ground, the inductor cur- voltage, the duty cycle increases toward the maximum rent decays very slowly during a single switching cycle. on-time. Further reduction of the supply voltage forces To prevent current runaway from occurring, a secondary the main switch to remain on for more than one cycle current limit is imposed on the inductor current. If the eventually reaching 100% duty cycle. The output voltage inductor valley current increases larger than 15A, the top will then be determined by the input voltage minus the power MOSFET will be held off and switching cycles will voltage drop across the internal P-channel MOSFET and be skipped until the inductor current is reduced. the inductor.
Voltage Tracking Low Supply Operation
Some microprocessors and DSP chips need two power The LTC3418 is designed to operate down to an input sup- supplies with different voltage levels. These systems often ply voltage of 2.25V. One important consideration at low require voltage sequencing between the core power sup- input supply voltages is that the RDS(ON) of the P-channel ply and the I/O power supply. Without proper sequencing, and N-channel power switches increases. The user should latch-up failure or excessive current draw may occur that calculate the power dissipation when the LTC3418 is used could result in damage to the processor’s I/O ports or the at 100% duty cycle with low input voltages to ensure that I/O ports of a supporting system device such as memory, thermal limits are not exceeded. an FPGA or a data converter. To ensure that the I/O loads are not driven until the core voltage is properly biased,
Slope Compensation and Inductor Peak Current
tracking of the core supply and the I/O supply voltage is Slope compensation provides stability in constant fre- necessary. quency architectures by preventing subharmonic oscilla- Voltage tracking is enabled by applying a ramp voltage tions at duty cycles greater than 50%. It is accomplished to the TRACK pin. When the voltage on the TRACK pin internally by adding a compensating ramp to the inductor is below 0.8V, the feedback voltage will regulate to this current signal. Normally, the maximum inductor peak tracking voltage. When the tracking voltage exceeds 0.8V, current is reduced when slope compensation is added. control over the feedback voltage is gradually released. In the LTC3418, however, slope compensation recovery Full release of tracking control over the feedback voltage is implemented to keep the maximum inductor peak cur- is achieved when the tracking voltage exceeds 1.05V. rent constant throughout the range of duty cycles. This keeps the maximum output current relatively constant
Voltage Reference Output
regardless of duty cycle. The LTC3418 provides a 1.25V reference voltage that is capable of sourcing up to 5mA of output current. This reference voltage is generated from a linear regulator and is intended for applications requiring a low noise reference voltage. To ensure that the output is stable, the reference voltage pin should be decoupled with a minimum of 2.2µF. 3418fc For more information www.linear.com/LTC3418 9 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts