Datasheet LTC1771 (Analog Devices) - 7

LTC1771
U U W U APPLICATIO S I FOR ATIO
The basic LTC1771 application circuit is shown in Figure where tOFF = 3.5µs. However, the ripple current at low 1 on the first page. External component selection is driven loads during Burst Mode operation is: by the load requirement and begins with the selection of ∆I R L(BURST) ≈ 35% of IPEAK ≈ 0.05/RSENSE SENSE. Once RSENSE is known, L can be chosen. Next, the MOSFET and D1 are selected. The inductor is chosen For best efficiency when Burst Mode operation is enabled, based largely on the desired amount of ripple current and choose: for Burst Mode operation. Finally CIN is selected for its ∆IL(CONT) ≤ ∆IL(BURST) ability to handle the required RMS input current and COUT is chosen with low enough ESR to meet the output voltage so that the inductor current is continuous during the burst ripple and transient specifications. periods. This sets a minimum inductor value of: LMIN = (75µH)(VOUT + VD)(RSENSE)
RSENSE Selection
When burst is disabled, ripple currents less than ∆IL(BURST) RSENSE is chosen based on the required output current. can be achieved by choosing L > LMIN. Lower ripple The LTC1771 current comparator has a maximum thresh- current reduces output voltage ripple and core losses, but old of 140mV/RSENSE. The current comparator threshold too low of ripple current will adversely effect efficiency. sets the peak inductor current, yielding a maximum aver- age output current IMAX equal to the peak less half the
Inductor Core Selection
peak-to-peak ripple current ∆IL. For best performance Once the value of L is known, the type of inductor must be when Burst Mode operation is enabled, choose ∆IL equal selected. High efficiency converters generally cannot to 35% of peak current. Allowing a margin for variations in afford the core loss found in low cost powdered iron the LTC1771 and external components gives the following cores, forcing the use of more expensive ferrite, equation for choosing RSENSE: molypermalloy or Kool Mµ® cores. Actual core loss is RSENSE = 100mV/IMAX independent of core size for a fixed inductor value, but is At higher supply voltages, the peak currents may be very dependent on inductance selected. As inductance slightly higher due to overshoot from current comparator increases, core losses go down. Unfortunately, increased delay and can be predicted from the second term in the inductance requires more turns of wire and therefore following equation: copper losses will increase. Ferrite designs have very low core loss and are preferred 1/2 0.14  V – V  at high switching frequencies, so design goals can con- I IN OUT PEAK ≅ + 0.5 R  L( H centrate on copper loss and preventing saturation. Ferrite SENSE µ )  core material saturates “hard,” which means that induc- tance collapses abruptly when the peak design current is
Inductor Value Selection
exceeded. This results in an abrupt increase in inductor Once RSENSE is known, the inductor value can be deter- ripple current and consequent increase in voltage ripple. mined. The inductance value has a direct effect on ripple Do not allow the core to saturate! current. The ripple current decreases with higher induc- Molypermalloy (from Magnetics, Inc.) is a very good, low tance and increases with higher VOUT. The ripple current loss core material for toroids, but it is more expensive than during continuous mode operation is set by the off-time ferrite. A reasonable compromise from the same manu- and inductance to be: facturer is Kool Mµ. Toroids are space efficient, especially  when you can use several layers of wire. Because they V V  ∆I OUT D L C ( ONT) = + tOFF generally lack a bobbin, mounting is more difficult. How-  L  ever, designs for surface mount are available that do not increase the height significantly. Kool Mµ is a registered trademark of Magnetics, Inc. 7