Datasheet LTC1707 (Analog Devices) - 8

LTC1707
U U W U APPLICATIO S I FOR ATIO
with the oscillator synchronized at its minimum frequency, forcing the use of more expensive ferrite, molypermalloy, i.e., to a clock just above the oscillator free-running or Kool Mµ® cores. Actual core loss is independent of core frequency. The actual reduction in average current is less size for a fixed inductor value, but it is very dependent on than for peak current. inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires The basic LTC1707 application circuit is shown in Figure␣ 1a. more turns of wire and therefore copper losses will External component selection is driven by the load re- increase. quirement and begins with the selection of L followed by CIN and COUT. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can con-
Inductor Value Calculation
centrate on copper loss and preventing saturation. Ferrite The inductor selection will depend on the operating fre- core material saturates “hard,” which means that induc- quency of the LTC1707. The internal preset frequency is tance collapses abruptly when the peak design current is 350kHz, but can be externally synchronized up to 550kHz. exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do The operating frequency and inductor selection are inter- not allow the core to saturate! related in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, oper- Kool Mµ (from Magnetics, Inc.) is a very good, low loss ating at a higher frequency generally results in lower core material for toroids with a “soft” saturation character- efficiency because of increased internal gate charge losses. istic. Molypermalloy is slightly more efficient at high (>200kHz) switching frequencies but quite a bit more The inductor value has a direct effect on ripple current. The expensive. Toroids are very space efficient, especially ripple current ∆IL decreases with higher inductance or when you can use several layers of wire, while inductors frequency and increases with higher VIN or VOUT. wound on bobbins are generally easier to surface mount. New designs for surface mount are available from 1   ∆ V I = V OUT L OUT 1 Coiltronics, Coilcraft and Sumida. f L  V ( )( ) − (1) IN 
C
Accepting larger values of ∆I
IN and COUT Selection
L allows the use of low inductances, but results in higher output voltage ripple In continuous mode, the source current of the top MOSFET and greater core losses. A reasonable starting point for is a square wave of duty cycle VOUT/VIN. To prevent large setting ripple current is ∆I voltage transients, a low ESR input capacitor sized for the L = 0.4(IMAX). maximum RMS current must be used. The maximum The inductor value also has an effect on Burst Mode RMS capacitor current is given by: operation. The transition to low current operation begins when the inductor current peaks fall to approximately / 200mA. Lower inductor values (higher ∆I V V [ ( −V )]1 2 L) will cause this OUT IN OUT to occur at lower load currents, which can cause a dip in C required I I IN RMS ≅ MAX V efficiency in the upper range of low current operation. In IN Burst Mode operation, lower inductance values will cause This formula has a maximum at VIN = 2VOUT, where the burst frequency to increase. IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations
Inductor Core Selection
do not offer much relief. Note that capacitor manufacturer’s Once the value for L is known, the type of inductor must be ripple current ratings are often based on 2000 hours of life. selected. High efficiency converters generally cannot This makes it advisable to further derate the capacitor, or afford the core loss found in low cost powdered iron cores, choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet Kool Mµ is a registered trademark of Magnetics, Inc. 8