Datasheet LTC3535 (Analog Devices) - 9

LTC3535
OPERATION (Refer to Block Diagram)
The LTC3535 is a dual channel 1MHz synchronous boost the output voltage exceeds the input by 0.24V, the channel converter housed in a 12-lead 3mm × 3mm DFN package. powers itself from VOUT instead of VIN. At this point the Each channel is identical and fully independent. They can internal circuitry has no dependency on the VIN input volt- operate from the same source, or from different voltage age, eliminating the requirement for a large input capacitor. sources. The input voltage can drop as low as 0.5V. The limiting In addition, their output voltages can be tied together factor for the application becomes the availability of the to allow operation of a single output from two different power source to supply sufficient energy to the output at input sources. However, note that the two channels are low voltages, and maximum duty cycle, which is clamped at not designed to current share, so if both input voltages 90% typical. Note that at low input voltages, small voltage are present either one may be supplying the load. drops due to series resistance become critical, and greatly limit the power delivery capability of the converter. The following description of operation applies to each channel. Note that references to VIN or VOUT apply to the
LOW NOISE FIXED FREQUENCY OPERATION
corresponding channel. With a guaranteed ability to start up and operate from
Soft-Start
inputs less than 0.8V, each channel features fixed fre- The LTC3535 contains internal circuitry to provide soft- quency, current mode PWM control for exceptional line start operation. The soft-start circuitry slowly ramps the and load regulation. The current mode architecture with peak inductor current from zero to its peak value of 750mA adaptive slope compensation provides excellent transient (typical) in approximately 0.5ms, allowing start-up into load response, requiring minimal output filtering. Internal heavy loads. The soft-start circuitry is reset in the event soft-start and internal loop compensation simplifies the of a shutdown command or a thermal shutdown. design process while minimizing the number of external components.
Oscillator
With its low RDS(ON) and low gate charge internal N-channel An internal oscillator (independent for each channel) sets MOSFET switch and P-channel MOSFET synchronous the switching frequency to 1MHz. rectifier, the LTC3535 achieves high efficiency over a wide range of load currents. Burst Mode operation maintains
Shutdown
high efficiency at very light loads, reducing the quiescent Shutdown is accomplished by pulling the SHDN pin below current to just 9µA per channel. Operation can be best 0.3V and enabled by pulling the SHDN pin above 0.8V. understood by referring to the Block Diagram. Although SHDN can be driven above VIN or VOUT (up to the absolute maximum rating) without damage, the LTC3535
LOW VOLTAGE START-UP
has a proprietary test mode that may be engaged if SHDN The LTC3535 includes an independent start-up oscillator is held in the range of 0.5V to 1V higher than the greater designed to start up at an input voltage of 0.68V (typical). of VIN or VOUT. If the test mode is engaged, normal PWM Soft-start and inrush current limiting are provided during switching action is interrupted, which can cause undesir- start-up, as well as normal mode. able operation in some applications. Therefore, in appli- cations where SHDN may be driven above V When either V IN, a resistor IN or VOUT for a given channel exceeds 1.3V divider or other means must be employed to keep the SHDN typical, the channel enters normal operating mode. When voltage below (VIN + 0.4V) to prevent the possibility of 3535fa 9 Document Outline Features Description Applcations Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts