Datasheet LT3740 (Analog Devices) - 10

LT3740
APPLICATIONS INFORMATION Choose MOSFET Sensing or Resistor Sensing
Besides I2R power loss, there are transition losses and gate drive losses. The transition losses that increase with The LT3740 can use either the bottom MOSFET on- the input voltage and inductor current are mainly in the resistance or an external sensing resistor for current top MOSFET. The losses can be estimated with a constant sensing. Simplicity and high effi ciency are the benefi ts k = 1.7A–1 as: of using bottom MOSFET on-resistance. However, some MOSFETs have a wide on-resistance variation. As discussed Transition Loss = k • V 2 IN • IL • CRSS • FS previously, the gate-source voltage and the temperature The gate drive losses increase with the gate drive power also affect the MOSFET on-resistance. These factors affect supply voltage, gate voltage and gate capacitance as the accuracy of the inductor current limit. The inductor shown below: saturation current will need enough margin to cover the current limit variation. In the cases where the input PGD,TOP = VBIAS • CGS,TOP • VGS,TOP • FS voltage supply has suffi cient current limit, a wide current PGD,BOT = VBGDP • CGS,BOT • VGS,BOT • FS limit variation of the controller may be tolerated. As the load increases to reach the input supply current limit, the
Duty Cycle Limits
input voltage corrupts, and limits the total power in the At the start of each oscillator cycle, the top MOSFET turns circuit. off and the bottom MOSFET turns on with a 500ns duty To reduce the current limit variation, a more accurate cycle on the top MOSFET. If the maximum duty cycle is external sensing resistor can be used between the bottom reached, due to a dropping input voltage for example, the MOSFET source and ground. Connect SN+ and SN– pins output voltage will droop out of regulation. to the two terminals of the resistor. Lower ripple current reduces core losses in the inductor,
Power Dissipation
ESR losses in the output capacitors and output voltage ripple. The highest effi ciency is obtained with a small The resulting power dissipation in the MOSFETs are: ripple current. However, achieving this requires a large P 2 inductor. There is a trade off between component size TOP = DTOP • IL • RDS(ON),TOP and effi ciency. P 2 BOT = DBOT • IL • RDS(ON),BOT A reasonable starting point is to choose a ripple current If an external sensing resistor is used, the extra power that is about 30% of IOUT(MAX). The largest ripple current dissipation in the sensing resistor is: occurs at the highest VIN. To guarantee that ripple current P 2 RS = DBOT • IL • Rs does not exceed a specifi ed maximum, the inductance should be chosen according to: The power losses in the bottom MOSFET and external sensing resistor are greatest during an output short-circuit, ⎛ V ⎞ ⎛ V ⎞ L = 1– OUT OUT where maximum inductor current and maximum bottom ⎜ ⎟ •⎜ ⎟ V ⎝ F • ⎝ ΔI IN(MAX) ⎠ S L(MAX) ⎠ duty cycle occur. 3740fc 10