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LTC3772EDDB Datasheet(PDF) 11 Page - Linear Technology |
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LTC3772EDDB Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page 11 LTC3772 3772f APPLICATIO S I FOR ATIO Inductor Core Selection Once the inductance value is determined, the type of induc- tor must be selected. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses abruptly when the peak design cur- rent is exceeded. This results in an abrupt increase in in- ductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko and Sumida. Output Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the diode conducts most of the time. As VIN approaches VOUT the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short- circuited. Under this condition the diode must safely handle IPEAK at close to 100% duty cycle. Therefore, it is impor- tant to adequately specify the diode peak current and av- erage power dissipation so as not to exceed the diode ratings. duty cycle–at its worst case the required RDS(ON) is given by: R P I DS ON DC P OUT MAX P ()( %) () () ( ) = = + 100 2 1 δ where PP is the allowable power dissipation and δP is the temperature dependency of RDS(ON). (1 + δP) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, but δP = 0.005/°C can be used as an approximation for low voltage MOSFETs. In applications where the maximum duty cycle is less than 100% and the LTC3772 is in continuous mode, the RDS(ON) is governed by: R P DC I DS ON P OUT P () () ( ) ≅ + 2 1 δ where DC is the maximum operating duty cycle of the LTC3772. Inductor Value Calculation The operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use of a smaller inductor for the same amount of inductor ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. The inductance value also has a direct effect on ripple current. The ripple current, IRIPPLE, decreases with higher inductance or frequency and increases with higher VIN or VOUT.Theinductor’speak-to-peakripplecurrentisgivenby: I VV fL VV VV RIPPLE IN OUT OUT D IN D = − + + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ () where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE occurs at the maximum input voltage. |
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