Datasheet LTC3803 (Analog Devices) - 10

LTC3803
APPLICATIONS INFORMATION CURRENT SENSE RESISTOR CONSIDERATIONS
Duty Cycle – 6% ΔV • 5μA •R The external current sense resistor (R SENSE = SL SENSE in Figure 2) 74% allows the user to optimize the current limit behavior for the particular application. As the current sense resistor Note: LTC3803 enforces 6% < Duty Cycle < 80%. is varied from several ohms down to tens of milliohms, A good starting value for RSL is 5.9k, which gives a 30mV peak switch current goes from a fraction of an ampere to drop in current comparator threshold at 80% duty cycle. several amperes. Care must be taken to ensure proper Designs not needing slope compensation may replace RSL circuit operation, especially with small current sense with a short circuit. resistor values. For example, a peak switch current of 5A requires a
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
sense resistor of 0.020Ω. Note that the instantaneous
LOCKOUT
peak power in the sense resistor is 0.5W and it must be The LTC3803 is designed to implement DC/DC converters rated accordingly. The LTC3803 has only a single sense operating from input voltages of typically 48V or more. line to this resistor. Therefore, any parasitic resistance The standard operating topology employs a third trans- in the ground side connection of the sense resistor will former winding (L increase its apparent value. In the case of a 0.020Ω sense BIAS in Figure 2) on the primary side that provides power for the LTC3803 via its V resistor, one milliohm of parasitic resistance will cause a CC pin. However, this arrangement is not inherently self-starting. Start-up is 5% reduction in peak switch current. So the resistance of affected by the use of an external “trickle-charge” resistor printed circuit copper traces and vias cannot necessarily (R be ignored. START in Figure 2) and the presence of an internal wide hysteresis undervoltage lockout circuit that monitors VCC pin voltage. Operation is as follows:
PROGRAMMABLE SLOPE COMPENSATION
“Trickle charge” resistor RSTART is connected to VIN and The LTC3803 injects a ramping current through its SENSE supplies a small current, typically on the order of 100μA pin into an external slope compensation resistor (RSL in to 120μA, to charge CVCC. After some time, the voltage Figure 2). This current ramp starts at zero right after the on CVCC reaches the VCC turn-on threshold. The LTC3803 NGATE pin has been high for the LTC3803’s minimum then turns on abruptly and draws its normal supply cur- duty cycle of 6%. The current rises linearly towards a rent. The NGATE pin begins switching and the external peak of 5μA at the maximum duty cycle of 80%, shutting MOSFET (Q1) begins to deliver power. The voltage on off once the NGATE pin goes low. A series resistor (RSL) CVCC begins to decline as the LTC3803 draws its normal connecting the SENSE pin to the current sense resistor supply current, which exceeds that delivered by RSTART. (RSENSE) thus develops a ramping voltage drop. From After some time, typically tens of milliseconds, the output the perspective of the SENSE pin, this ramping voltage voltage approaches its desired value. By this time, the third adds to the voltage across the sense resistor, effectively transformer winding is providing virtually all the supply reducing the current comparator threshold in proportion current required by the LTC3803. to duty cycle. This stabilizes the control loop against subharmonic oscillation. The amount of reduction in the One potential design pitfall is undersizing the value of current comparator threshold (ΔV capacitor CVCC. In this case, the normal supply current SENSE) can be calculated using the following equation: drawn by the LTC3803 will discharge CVCC too rapidly; before the third winding drive becomes effective, the VCC turn-off threshold will be reached. The LTC3803 turns off, 3803fc 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 APPLICATIONS RELATED PARTS