Datasheet LTC3805 (Analog Devices) - 10

LTC3805
operation
First, consider operation without overcurrent protection. Add a capacitor CSS from the SSFLT pin to GND to increase For some maximum converter output current, the voltage both the soft-start time and the time for fault timeout. Dur- on the ITH pin rises to and is clamped at approximately 1.9V. ing soft-start, CSS is charged with a 6µA current. When the This corresponds to a 100mV limit on the voltage at the LTC3805 comes out of shutdown, the LTC3805 quickly ISENSE pin. As the output current is further increased, the charges CSS to about 0.7V at which point GATE begins duty cycle is reduced as the output voltage sags. However, switching. From that point, GATE continues switching with the peak current in the external MOSFET is limited by the increasing duty cycle until the SSFLT pin reaches about 100mV threshold at the ISENSE pin. 2.25V at which point soft-start is over and closed-loop As the output current is increased further, eventually, regulation begins. The voltage on the SSFLT pin addition- the duty cycle is reduced to the 6% minimum. Since the ally further charges to about 4.75V. external MOSFET is always turned on for this minimum CSS also performs the timeout function in the event of a amount of time, the current comparator no longer limits fault. After a fault, CSS is slowly discharged from about the current through the external MOSFET based on the 4.75V to about 0.7V by a 2µA current. When the voltage 100mV threshold. If the output current continues to in- on the SSFLT pin reaches 0.7V the converter attempts to crease, the current through the MOSFET could rise to a restart. More detail on programming the external soft-start level that would damage the converter. fault timeout is described in the Applications Information To prevent damage, the overcurrent pin, OC, is also section. connected to the current sense resistor, and a fault is
Powering the LTC3805
triggered if the voltage on the OC pin exceeds 100mV. To protect itself, the converter stops operating as described A built-in shunt regulator from the VCC pin to GND limits in the next section. External resistors can be used to ad- the voltage on the VCC pin to approximately 9.5V as long just the overcurrent threshold to voltages higher or lower as the shunt regulator is not forced to sink more than than 100mV as described in the Applications Information 25mA. The shunt regulator is always active, even when section. the LTC3805 is in shutdown, since it serves the vital func- tion of protecting the VCC pin from overvoltage. The shunt
Soft-Start and Fault Timeout Operation
regulator permits the use of a wide variety of powering schemes for the LTC3805 even from high voltage sources The soft-start and fault timeout of the LTC3805 uses either that exceed the LTC3805’s absolute maximum ratings. a fixed internal timer or an external timer programmed Further details on powering schemes are described in the by a capacitor from the SSFLT pin to GND. The internal Applications Information section. soft-start and fault timeout times are minimums and can be increased by placing a capacitor from the SSFLT pin
Adjustable Slope Compensation
to GND. Operation is shown in Figure 1. The LTC3805 injects a 10µA peak current ramp out of Leave the SSFLT pin open to use the internal soft-start and its ISENSE pin which can be used, in conjunction with an fault timeout. The internal soft-start is complete in about external resistor, for slope compensation in designs that 1.8ms. In the event of an overcurrent as detected by the require it. This current ramp is approximately linear and OC pin exceeding 100mV, the LTC3805 shuts down and begins at zero current at 6% duty cycle, reaching peak an internal timing circuit waits for a fault timeout of about current at 80% duty cycle. Additional details are provided 4.25ms and then restarts the converter. in the Applications Information section. 3805fg 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration 0rder Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts