Datasheet LT3494, LT3494A (Analog Devices) - 7

LT3494/LT3494A
OPERATION
The LT3494/LT3494A use a novel control scheme to pro- Because the switching frequency is never allowed to fall vide high effi ciency over a wide range of output current. below approximately 50kHz, a minimum load must be In addition, this technique keeps the switching frequency present to prevent the output voltage from drifting too high. above the audio band over all load conditions. This minimum load is automatically generated within the part via the Shunt Control block. The level of this current The operation of the part can be better understood by is adaptable, removing itself when not needed to improve refering to the Block Diagram. The part senses the output effi ciency at higher load levels. voltage by monitoring the voltage on the FB pin. The user sets the desired output voltage by choosing the value of The LT3494/LT3494A also have an integrated Schottky the external top feedback resistor. The parts incorporate diode and PMOS output disconnect switch. The PMOS a precision 182k bottom feedback resistor. Assuming that switch is turned on when the part is enabled via the SHDN output voltage adjustment is not used (CTRL pin is tied to pin. When the parts are in shutdown, the PMOS switch 1.5V or greater), the internal reference (VREF = 1.225V) sets turns off, allowing the VOUT node to go to ground. This the voltage at which FB will servo to during regulation. type of disconnect function is often required in power supplies. The Switch Control block senses the output of the ampli- fi er and adjusts the switching frequency as well as other The only difference between the LT3494A and LT3494 parameters to achieve regulation. During the start-up of is the level of the current limit. The LT3494A has a typi- the circuit, special precautions are taken to insure that the cal peak current limit of 350mA while the LT3494 has a inductor current remains under control. 180mA limit.
APPLICATIONS INFORMATION Choosing an Inductor
This value provides a good trade off in inductor size and Several recommended inductors that work well with the system performance. Pick a standard inductor close to LT3494/LT3494A are listed in Table 1, although there are this value. A larger value can be used to slightly increase many other manufacturers and devices that can be used. the available output current, but limit it to around twice Consult each manufacturer for more detailed information the value calculated below, as too large of an inductance and for their entire selection of related parts. Many dif- will decrease the output voltage ripple without providing ferent sizes and shapes are available. Use the equations much additional output current. A smaller value can be and recommendations in the next few sections to fi nd the used (especially for systems with output voltages greater correct inductance value for your design. than 12V) to give a smaller physical size. Inductance can be calculated as:
Inductor Selection—Boost Regulator
L = (VOUT – VIN(MIN) + 0.5V) • 0.66 (μH) The formula below calculates the appropriate inductor where VOUT is the desired output voltage and VIN(MIN) is value to be used for a boost regulator using the LT3494/ the minimum input voltage. Generally, a 10μH or 15μH LT3494A (or at least provides a good starting point). inductor is a good choice.
Table 1. Recommended Inductors VALUE MAX DCR MAX DC I SIZE PART FOR USE WITH (μH) (
Ω
) (mA) (mm
×
mm
×
mm) VENDOR
LQH32CN100K53 LT3494/LT3494A 10 0.3 450 3.5 × 2.7 × 1.7 Murata LQH32CN150K53 LT3494 15 0.58 300 3.5 × 2.7 × 1.7 www.murata.com CDRH3D11-100 LT3494 10 0.24 280 4.0 × 4.0 × 1.2 Sumida CDHED13/S-150 LT3494/LT3494A 15 0.55 550 4.0 × 4.2 × 1.4 www.sumida.com 3494fb 7