Datasheet LTC1622 (Analog Devices) - 8

LTC1622
U U W U APPLICATIONS INFORMATION
preferred at high switching frequencies, so design goals In applications where the maximum duty cycle is less than can concentrate on copper loss and preventing saturation. 100% and the LTC1622 is in continuous mode, the RDS(ON) Ferrite core materials saturate “hard,” which means that is governed by: the inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in P R P DS ON ( ) ≅ inductor ripple current and consequently, output voltage 2 DC I 1 p ( ) ( +δ ) ripple. Do not allow the core to saturate! 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 than LTC1622. ferrite. A reasonable compromise from the same manu- When the LTC1622 is operating in continuous mode, the facturer is Kool Mu. Toroids are very space efficient, MOSFET power dissipation is: especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more V V 2 difficult. However, new surface mountable designs that do P OUT D = + I (1+ pδ R ( ) MOSFET OUT DS O ( N) not increase the height significantly are available. V + V IN D ( )2( )( )( )
Power MOSFET Selection
+K V I C f IN OUT RSS An external P-channel power MOSFET must be selected where K is a constant inversely related to gate drive for use with the LTC1622. The main selection criteria for current. Because of the high switching frequency, the the power MOSFET are the threshold voltage VGS(TH) and second term relating to switching loss is important not to the “on” resistance RDS(ON),reverse transfer capacitance overlook. The constant K = 3 can be used to estimate the CRSS and total gate charge. contributions of the two terms in the MOSFET dissipation Since the LTC1622 is designed for operation down to low equation. input voltages, a sublogic level threshold MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required for applications that
Output Diode Selection
work close to this voltage. When these MOSFETs are used, The catch diode carries load current during the off-time. make sure that the input supply to the LTC1622 is less than The average diode current is therefore dependent on the the absolute maximum MOSFET VGS rating, typically 8V. P-channel switch duty cycle. At high input voltages the The gate drive voltage levels are from ground to VIN. diode conducts most of the time. As VIN approaches VOUT The required minimum RDS(ON) of the MOSFET is gov- the diode conducts only a small fraction of the time. The erned by its allowable power dissipation. For applications most stressful condition for the diode is when the output that may operate the LTC1622 in dropout, i.e., 100% duty is short circuited. Under this condition the diode must cycle, at its worst case the required RDS(ON) is given by: safely handle IPEAK at close to 100% duty cycle. Therefore, it is important to adequately specify the diode peak current P R P DS O ( N) = and average power dissipation so as not to exceed the DC= % 100 2 I (1+ p ( ) δ ) OUT MAX ( ) diode ratings. Under normal load conditions, the average current con- where PP is the allowable power dissipation and δp is the ducted by the diode is: temperature dependency of RDS(ON). (1 + δp) is generally given for a MOSFET in the form of a normalized RDS(ON) vs  V V  temperature curve, but δp = 0.005/°C can be used as an I IN OUT = − I D OUT approximation for low voltage MOSFETs.  V + V IN D  8