Datasheet LTC3803-3 (Analog Devices) - 10

LTC3803-3
APPLICATIONS INFORMATION CURRENT SENSE RESISTOR CONSIDERATIONS
current comparator threshold (ΔVSENSE) can be calculated The external current sense resistor (R using the following equation: SENSE in Figure 2) allows the user to optimize the current limit behavior for Duty Cycle – 8% • 5 the particular application. As the current sense resistor ΔV μA • R SENSE = SL 80% is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to Note: LTC3803-3 enforces 8% < Duty Cycle < 80%. several amperes. Care must be taken to ensure proper A good starting value for RSL is 5.9k, which gives a 30mV circuit operation, especially with small current sense drop in current comparator threshold at 80% duty cycle. resistor values. Designs not needing slope compensation may replace RSL For example, a peak switch current of 5A requires a sense with a short circuit. resistor of 0.020Ω. Note that the instantaneous peak power in the sense resistor is 0.5W and it must be rated
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
accordingly. The LTC3803-3 has only a single sense line
LOCKOUT
to this resistor. Therefore, any parasitic resistance in the The LTC3803-3 is designed to implement DC/DC converters ground side connection of the sense resistor will increase operating from input voltages of typically 48V or more. The its apparent value. In the case of a 0.020Ω sense resis- standard operating topology employs a third transformer tor, one milliohm of parasitic resistance will cause a 5% winding (L reduction in peak switch current. So the resistance of BIAS in Figure 2) on the primary side that pro- vides power for the LTC3803-3 via its V printed circuit copper traces and vias cannot necessarily CC pin. However, this arrangement is not inherently self-starting. Start-up be ignored. is affected by the use of an external trickle-charge resistor (RSTART in Figure 2) and the presence of an internal wide
PROGRAMMABLE SLOPE COMPENSATION
hysteresis undervoltage lockout circuit that monitors VCC The LTC3803-3 injects a ramping current through its SENSE pin voltage. Operation is as follows: pin into an external slope compensation resistor (RSL in Trickle-charge resistor RSTART is connected to VIN and Figure 2). This current ramp starts at zero right after the supplies a small current, typically on the order of 100μA, NGATE pin has been high for the LTC3803-3’s minimum to charge CVCC. After some time, the voltage on CVCC duty cycle of 8%. The current rises linearly towards a reaches the VCC turn-on threshold. The LTC3803-3 then peak of 5μA at the maximum duty cycle of 80%, shutting turns on abruptly and draws its normal supply current. off once the NGATE pin goes low. A series resistor (RSL) The NGATE pin begins switching and the external MOSFET connecting the SENSE pin to the current sense resistor (Q1) begins to deliver power. The voltage on CVCC begins (RSENSE) thus develops a ramping voltage drop. From to decline as the LTC3803-3 draws its normal supply the perspective of the SENSE pin, this ramping voltage current, which exceeds that delivered by RSTART . After adds to the voltage across the sense resistor, effectively some time, typically tens of milliseconds, the output volt- reducing the current comparator threshold in proportion age approaches its desired value. By this time, the third to duty cycle. This stabilizes the control loop against transformer winding is providing virtually all the supply subharmonic oscillation. The amount of reduction in the current required by the LTC3803-3. 38033fd 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS