Datasheet LTC3539, LTC3539-2 (Analog Devices) - 7

LTC3539/LTC3539-2
OPERATION DETAILED DESCRIPTION
to series resistance become critical, and greatly limit the The LTC3539 is a high power synchronous boost converter power delivery capability of the converter. in an 8-lead 2mm × 3mm DFN package. With the ability
Low Noise Fixed Frequency Operation
to start up and operate from inputs of 700mV, the device features fixed frequency, current mode PWM control for Soft-Start: The LTC3539/LTC3539-2 contains internal exceptional line and load regulation. The current mode circuitry to provide soft-start operation. The internal architecture with adaptive slope compensation provides soft-start circuitry ramps the peak inductor current from excellent transient load response, requiring minimal output zero to its peak value of 2.6A (typical) in approximately filtering. Internal soft-start and internal loop compensation 0.5ms, allowing start-up into heavy loads. The soft-start simplifies the design process while minimizing the number circuitry is reset in the event of a thermal shutdown or of external components. The switching frequency of the shutdown command. LTC3539 is nominal y 1MHz, while the LTC3539-2 switches Oscillator: An internal oscillator sets the switching fre- at 2MHz. Operation of the LTC3539 and LTC3539-2 is quency to 1MHz for the LTC3539, and 2MHz for the identical in all other respects. LTC3539-2. With its low RDS(ON) and low gate charge internal N-channel Shutdown: The part is shutdown by pulling the SHDN MOSFET switch and P-channel MOSFET synchronous pin below 0.3V, and activated by pulling the SHDN pin rectifier, the LTC3539 achieves high efficiency over a wide above 0.88V. Note that SHDN can be driven above V range of load current. Burst Mode operation maintains IN or V high efficiency at very light loads, reducing the quiescent OUT, as long as it is limited to less than the absolute maximum rating. current to just 10µA. Error Amplifier: The error amplifier is a transconductance Converter operation can be best understood by referring type. The non-inverting input is internally connected to to the Block Diagram. the 1.2V reference and the inverting input is connected to FB. Clamps limit the minimum and maximum error
Low Voltage Start-Up
amp output voltage for improved large signal transient The LTC3539 includes an independent start-up oscillator response. Power converter control loop compensation is designed to start up at an input voltage of 0.7V (typical). provided internally. A voltage divider from VOUT to ground Soft-start and inrush current limiting are provided during programs the output voltage via FB from 1.5V to 5.25V. start-up, as well as normal mode. VOUT = 1.2V • [1 + (R2/R1)]. When either VIN or VOUT exceeds 1.4V typical, the IC Current Sensing: Lossless current sensing converts the enters normal operating mode. Once the output voltage peak current signal of the N-channel MOSFET switch exceeds the input by 0.24V, the IC powers itself from into a voltage which is summed with the internal slope VOUT instead of VIN. At this point the internal circuitry has compensation. The summed signal is compared to the no dependency on the VIN input voltage, eliminating the error amplifier output to provide a peak current control requirement for a large input capacitor. The input voltage command for the PWM. can drop as low as 0.5V. The limiting factor for the appli- Current Limit: The current limit comparator shuts off the cation becomes the ability of the power source to supply N-channel MOSFET switch once its threshold is reached. sufficient energy to the output at the low voltages, and the Peak switch current is limited to approximately 2.6A, maximum duty cycle, which is clamped at 90% typical. independent of input or output voltage, unless V Note that at low input voltages, small voltage drops due OUT falls below 0.7V, in which case the current limit is cut in half. 35392fc For more information www.linear.com/3539/3539-2 7 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 Package Description Revision History Related Parts