Datasheet LTC1773 (Analog Devices) - 8

LTC1773
U U W U APPLICATIONS INFORMATION
The operating frequency and inductor selection are inter- inductor ripple current and consequent output voltage related in that higher operating frequencies allow the use ripple. Do not allow the core to saturate! of smaller inductor and capacitor values. However, oper- Molypermalloy (from Magnetics, Inc.) is a very good, low ating at a higher frequency generally results in lower loss core material for toroids, but it is more expensive than efficiency because of external MOSFET gate charge losses. ferrite. A reasonable compromise from the same manu- The inductor value has a direct effect on ripple current. The facturer is Kool Mµ. Toroids are very space efficient, ripple current, ∆IL, decreases with higher inductance or especially when you can use several layers of wire. Be- frequency and increases with higher VIN or VOUT. cause they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount are 1 ⎛ V ⎞ available which do not increase the height significantly. ∆I = V OUT L OUT 1– (1) f L ⎝⎜ V ( )( ) IN ⎠ ⎟
Power MOSFET and Schottky Diode Selection
Accepting larger values of ∆I Two external power MOSFETs must be selected for use L allows the use of lower inductances, but results in higher output voltage ripple with the LTC1773: a P-channel MOSFET for the top (main) and greater core losses. A reasonable starting point for switch, and an N-channel MOSFET for the bottom (syn- setting ripple current is 30% to 40% of I chronous) switch. MAX. Remember, the maximum ∆IL occurs at the maximum input voltage. The peak-to-peak gate drive levels are set by the VIN The inductor value also has an effect on Burst Mode voltage. Therefore, for VIN > 5V, logic-level threshold operation. The transition to low current operation begins MOSFETs should be used. But, for VIN < 5V, sub-logic when the inductor current peaks fall to approximately 1/3 level threshold MOSFETs (VGS(TH) < 3V) should be used. its original value. Lower inductor values (higher ∆I In these applications, make sure that the V L) will IN to the cause this to occur at lower load currents, which can cause LTC1773 is less than 8V because the absolute maximum a dip in efficiency in the upper range of low current VGS rating of the majority of these sub-logic threshold operation. In Burst Mode operation, lower inductance MOSFETs is 8V. values will cause the burst frequency to increase. Selection criteria for the power MOSFETs include the “ON” resistance RDS(ON), reverse transfer capacitance CRSS,
Inductor Core Selection
input voltage, maximum output current, and total gate Once the value for L is known, the type of inductor must be charge. When the LTC1773 is operating in continuous selected. High efficiency converters generally cannot af- mode the duty cycles for the top and bottom MOSFETs are ford the core loss found in low cost powdered iron cores, given by: forcing the use of more expensive ferrite, molypermalloy, Main Switch Duty Cycle = VOUT/VIN or Kool Mµ® cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN inductance selected. As inductance increases, core losses The MOSFET power dissipations at maximum output go down. Unfortunately, increased inductance requires current are given by: more turns of wire and therefore copper losses will in- crease. Ferrite designs have very low core losses and are V 2 preferred at high switching frequencies, so design goals P OUT = I ( ) (1+ δ R) MAIN MAX DSON + VIN can concentrate on copper loss and preventing saturation. 2 Ferrite core material saturates “hard”, which means that K(V ) I( )(C )(f) IN MAX RSS inductance collapses abruptly when the peak design cur- rent is exceeded. This results in an abrupt increase in 1773fb 8