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LTC3526LEDC-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC3526LEDC-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LTC3526L/LTC3526LB 8 3526lfb The LTC3526L/LTC3526LB are 1MHz synchronous boost converters housed in a 6-lead 2mm × 2mm DFN package. With a guaranteed ability to start up and operate from inputs less than 0.8V, this device features fixed frequency, current mode PWM control for exceptional line and load regulation. The current mode architecture with adaptive slope compensation provides excellent transient load response, requiring minimal output filtering. Internal soft-start and internal loop compensation simplifies the design process while minimizing the number of external components. With its low RDS(ON) and low gate charge internal N-chan- nel MOSFET switch and P-channel MOSFET synchronous rectifier, the LTC3526L achieves high efficiency over a wide range of load currents. Burst Mode operation maintains high efficiency at very light loads, reducing the quiescent current to just 9μA. Operation can be best understood by referring to the Block Diagram. LOW VOLTAGE START-UP The LTC3526L/LTC3526LB include an independent start- up oscillator designed to start up at an input voltage of 0.68V (typical). Soft-start and inrush current limiting are provided during start-up, as well as normal mode. When either VIN or VOUT exceeds 1.3V typical, the IC enters normal operating mode. When the output voltage exceeds the input by 0.24V, the IC powers itself from VOUT instead of VIN. At this point the internal circuitry has no dependency on the VIN input voltage, eliminating the requirement for a large input capacitor. The input voltage can drop as low as 0.5V. The limiting factor for the ap- plication becomes the availability of the power source to supply sufficient energy to the output at low voltages, and maximum duty cycle, which is clamped at 90% typical. Note that at low input voltages, small voltage drops due to series resistance become critical, and greatly limit the power delivery capability of the converter. LOW NOISE FIXED FREQUENCY OPERATION Soft-Start The LTC3526L/LTC3526LB contain internal circuitry to provide soft-start operation. The soft-start circuitry slowly ramps the peak inductor current from zero to its peak value of 750mA (typical) in approximately 0.5ms, allowing start- up into heavy loads. The soft-start circuitry is reset in the event of a shutdown command or a thermal shutdown. Oscillator An internal oscillator sets the switching frequency to 1MHz. Shutdown Shutdown is accomplished by pulling the ⎯S⎯H⎯D⎯N pin below 0.3V and enabled by pulling the ⎯S⎯H⎯D⎯N pin above 0.8V. Note that ⎯S⎯H⎯D⎯N can be driven above VIN or VOUT, as long as it is limited to less than the absolute maximum rating. Error Amplifier The positive input of the transconductance error amplifier is internally connected to the 1.195V reference and the negative input is connected to FB. Clamps limit the mini- mum and maximum error amp output voltage for improved large-signal transient response. Power converter control loop compensation is provided internally. An external resistive voltage divider from VOUT to ground programs the output voltage via FB from 1.5V to 5.25V. VOUT =1.195V • 1+ R2 R1 Current Sensing Lossless current sensing converts the peak current signal of the N-channel MOSFET switch into a voltage that is summed with the internal slope compensation. The summed signal is compared to the error amplifier output to provide a peak current control command for the PWM. Current Limit The current limit comparator shuts off the N-channel MOSFET switch once its threshold is reached. The cur- rent limit comparator delay to output is typically 60ns. Peak switch current is limited to approximately 750mA, independent of input or output voltage, unless VOUT falls below 0.7V, in which case the current limit is cut in half. (Refer to Block Diagram) OPERATION |
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