Datasheet LTC3672B-1 (Analog Devices) - 8

LTC3672B-1
OPERATION LOW DROPOUT LINEAR REGULATORS (LDOS)
1.2V/3.6V = 33.3%, using the fact that the upper-bound on any linear regulator’s effi ciency is output voltage divided by The LTC3672B-1 contains two 150mA fi xed-output LDO input voltage. Feeding the LDO from the output of the buck regulators. LDO1 takes power from the VIN1 pin and regu- regulator, with a typical buck effi ciency of 85%, raises the lates a 1.2V output at the LDO1 pin. By connecting VIN1 ceiling on overall effi ciency to 85% • 1.2V/1.8V = 56.6%. to the buck regulator’s 1.8V output, overall conversion This can increase battery life by up to 70%! effi ciency can be improved, because the bulk of the step- down will be done by the buck regulator at higher effi ciency LDO2 takes power straight from VIN and regulates a 2.8V than what the LDO can do on its own. For example, for output at the LDO2 pin. the case of deriving a 1.2V output from a 3.6V input (e.g. For stability, each LDO output must be bypassed to ground Lithium-Ion battery nominal voltage), using an LDO to with a minimum 1μF ceramic capacitor. do all of the step-down results in an effi ciency of at most
APPLICATIONS INFORMATION BUCK REGULATOR INDUCTOR SELECTION
Different core materials and shapes will change the size/cur- rent and price/current relationship of an inductor. Toroid Many different sizes and shapes of inductors are avail- or shielded pot cores in ferrite or Permalloy™ materials able from numerous manufacturers. Choosing the right are small and don’t radiate much energy, but generally inductor from such a large selection of devices can be cost more than powdered iron core inductors with similar overwhelming, but following a few basic guidelines will electrical characteristics. Inductors that are very thin or make the selection process much simpler. have a very small volume typically have much higher core The buck regulator is designed to work with inductors in and DCR losses, and will not give the best effi ciency. The the range of 2.2μH to 10μH. A 4.7μH inductor is a good choice of which style inductor to use often depends more starting point. Larger value inductors reduce ripple cur- on the price vs size, performance, and any radiated EMI rent, which improves output ripple voltage. Lower value requirements than on what the buck regulator needs to inductors result in higher ripple current and improved operate. transient response time. To maximize effi ciency, choose Table 1 shows several inductors that work well with the an inductor with a low DC resistance. Choose an inductor buck regulator. These inductors offer a good compromise with a DC current rating at least 1.5 times larger than the in current rating, DCR and physical size. Consult each maximum load current to ensure that the inductor does manufacturer for detailed information on their entire not saturate during normal operation. If output short circuit selection of inductors. is a possible condition, the inductor should be rated to handle the maximum peak current specifi ed for the step- down converters. 3672B1f 8