Datasheet LTC3803-5 (Analog Devices) - 10

LTC3803-5
APPLICATIONS INFORMATION Leakage Inductance
For example, a peak switch current of 5A requires a sense resistor of 0.020Ω. Note that the instantaneous peak power Transformer leakage inductance (on either the primary in the sense resistor is 0.5W and it must be rated accord- or secondary) causes a voltage spike to occur after the ingly. The LTC3803-5 has only a single sense line to this output switch (Q1) turn-off. This is increasingly prominent resistor. Therefore, any parasitic resistance in the ground at higher load currents, where more stored energy must side connection of the sense resistor will increase its ap- be dissipated. In some cases a “snubber” circuit will be parent value. In the case of a 0.020Ω sense resistor, one required to avoid overvoltage breakdown at the MOSFET’s milliohm of parasitic resistance will cause a 5% reduction drain node. Application Note 19 is a good reference on in peak switch current. So the resistance of printed circuit snubber design. copper traces and vias cannot necessarily be ignored. A bifi lar or similar winding technique is a good way to minimize troublesome leakage inductances. However,
PROGRAMMABLE SLOPE COMPENSATION
remember that this will limit the primary-to-secondary breakdown voltage, so bifi lar winding is not always The LTC3803-5 injects a ramping current through its SENSE practical. pin into an external slope compensation resistor (RSL in Figure 2). This current ramp starts at zero right after the NGATE pin has been high for the LTC3803-5’s minimum
CURRENT SENSE RESISTOR CONSIDERATIONS
duty cycle of 6.5%. The current rises linearly towards a The external current sense resistor (RSENSE in Figure 2) peak of 5μA at the maximum duty cycle of 80%, shutting allows the user to optimize the current limit behavior for off once the NGATE pin goes low. A series resistor (RSL) the particular application. As the current sense resistor connecting the SENSE pin to the current sense resistor is varied from several ohms down to tens of milliohms, (RSENSE) thus develops a ramping voltage drop. From peak switch current goes from a fraction of an ampere to the perspective of the SENSE pin, this ramping voltage several amperes. Care must be taken to ensure proper adds to the voltage across the sense resistor, effectively circuit operation, especially with small current sense reducing the current comparator threshold in proportion resistor values. 38035fd 10 Document Outline FEATURES DESCRIPTION APPLICATIONS 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 TYPICAL APPLICATION RELATED PARTS