Datasheet LTC3801, LTC3801B (Analog Devices) - 8

LTC3801/LTC3801B
U U W U APPLICATIO S I FOR ATIO
IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE manufacturer is Kool Mµ. Toroids are very space efficient, occurs at the maximum input voltage. especially when you can use several layers of wire. In Burst Mode operation on the LTC3801, the ripple Because they generally lack a bobbin, mounting is more current is normally set such that the inductor current is difficult. However, new designs for surface mount that do continuous during the burst periods. Therefore, the peak- not increase the height significantly are available. to-peak ripple current must not exceed:
Power MOSFET Selection
. An external P-channel power MOSFET must be selected IRIPPLE≤ 0 03 RSENSE for use with the LTC3801/LTC3801B. The main selection criteria for the power MOSFET are the threshold voltage This implies a minimum inductance of: VGS(TH) and the “on” resistance RDS(ON), reverse transfer V V  V + V  capacitance CRSS and total gate charge. L IN OUT OUT D MIN = −  .   V + V  0 03 Since the LTC3801/LTC3801B are designed for operation f IN D R down to low input voltages, a sublogic level threshold SENSE   MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required (Use VIN(MAX) = VIN) for applications that work close to this voltage. When A smaller value than L these MOSFETs are used, make sure that the input supply MIN could be used in the circuit; however, the inductor current will not be continuous to the LTC3801/LTC3801B is less than the absolute maxi- during burst periods. mum VGS rating, typically 8V. The required minimum RDS(ON) of the MOSFET is governed
Inductor Core Selection
by its allowable power dissipation. For applications that may Once the value for L is known, the type of inductor must be operate the LTC3801/LTC3801B in dropout, i.e., 100% duty selected. High efficiency converters generally cannot af- cycle, at its worst case the required RDS(ON) is given by: ford the core loss found in low cost powdered iron cores, P forcing the use of more expensive ferrite, molypermalloy R P DS ON ( = DC ) = % 100 2 or Kool Mµ® cores. Actual core loss is independent of core I ( ) (1+ pδ) OUT MAX ( ) size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses where PP is the allowable power dissipation and δp is the go down. Unfortunately, increased inductance requires temperature dependency of RDS(ON). (1 + δp) is generally more turns of wire and therefore copper losses will in- given for a MOSFET in the form of a normalized RDS(ON) vs crease. Ferrite designs have very low core losses and are temperature curve, but δp = 0.005/°C can be used as an preferred at high switching frequencies, so design goals approximation for low voltage MOSFETs. can concentrate on copper loss and preventing saturation. In applications where the maximum duty cycle is less than Ferrite core material saturates “hard,” which means that 100% and the LTC3801/LTC3801B are in continuous inductance collapses abruptly when the peak design cur- mode, the RDS(ON) is governed by: rent is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage P R P DS ON ( ) ≅ ripple. Do not allow the core to saturate! DC ( )I 2(1+ p δ ) OUT Molypermalloy (from Magnetics, Inc.) is a very good, low where DC is the maximum operating duty cycle of the loss core material for toroids, but it is more expensive LTC3801/LTC3801B. than ferrite. A reasonable compromise from the same Kool Mµ is a registered trademark of Magnetics, Inc. sn3801 3801fs 8