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LTC3805EDD-5-TRMPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3805EDD-5-TRMPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3805-5 10 38055fa OPERATION First, consider operation without overcurrent protection. For some maximum converter output current, the voltage on the ITH pin rises to and is clamped at approximately 1.9V. This corresponds to a 100mV limit on the voltage at the ISENSE pin. As the output current is further increased, the duty cycle is reduced as the output voltage sags. However, the peak current in the external MOSFET is limited by the 100mV threshold at the ISENSE pin. As the output current is increased further, eventually, the duty cycle is reduced to the 6% minimum. Since the external MOSFET is always turned on for this minimum amount of time, the current comparator no longer limits the current through the external MOSFET based on the 100mV threshold. If the output current continues to in- crease, the current through the MOSFET could rise to a level that would damage the converter. To prevent damage, the overcurrent pin OC is also connected to the current sense resistor, and a fault is triggered if the voltage on the OC pin exceeds 100mV. To protect itself, the converter stops operating as described in the next section. External resistors can be used to ad- just the overcurrent threshold to voltages higher or lower than 100mV as described in the Applications Information section. Soft-Start and Fault Timeout Operation The soft-start and fault timeout of the LTC3805-5 uses either a fixed internal timer or an external timer programmed by a capacitor from the SSFLT pin to GND. The internal soft-start and fault timeout times are minimums and can be increased by placing a capacitor from the SSFLT pin to GND. Operation is shown in Figure 1. Leave the SSFLT pin open to use the internal soft-start and fault timeout. The internal soft-start is complete in about 1.8ms. In the event of an overcurrent as detected by the OC pin exceeding 100mV, the LTC3805-5 shuts down and an internal timing circuit waits for a fault timeout of about 4.25ms and then restarts the converter. Add a capacitor CSS from the SSFLT pin to GND to increase both the soft-start time and the time for fault timeout. Dur- ing soft-start, CSS is charged with a 6μA current. When the LTC3805-5 comes out of shutdown, the LTC3805-5 quickly charges CSS to about 0.7V at which point GATE begins switching. From that point, GATE continues switching with increasing duty cycle until the SSFLT pin reaches about 2.25V at which point soft-start is over and closed-loop regulation begins. The voltage on the SSFLT pin additionally further charges to about 4.75V. CSS also performs the timeout function in the event of a fault. After a fault, CSS is slowly discharged from about 4.75V to about 0.7V by a 2μA current. When the voltage on the SSFLT pin reaches 0.7V the converter attempts to restart. More detail on programming the external soft-start fault timeout is described in the Applications Information section. Powering the LTC3805-5 A built-in shunt regulator from the VCC pin to GND limits the voltage on the VCC pin to approximately 9.5V as long as the shunt regulator is not forced to sink more than 25mA. The shunt regulator is always active, even when the LTC3805-5 is in shutdown, since it serves the vital function of protecting the VCC pin from overvoltage. The shunt regulator permits the use of a wide variety of power- ing schemes for the LTC3805-5 even from high voltage sources that exceed the LTC3805-5’s absolute maximum ratings. Further details on powering schemes are described in the Applications Information section. Adjustable Slope Compensation The LTC3805-5 injects a 10μA peak current ramp out of its ISENSE pin which can be used, in conjunction with an external resistor, for slope compensation in designs that require it. This current ramp is approximately linear and begins at zero current at 6% duty cycle, reaching peak current at 80% duty cycle. Additional details are provided in the Applications Information section. |
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