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LTC1174IS8 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1174IS8 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LTC1174 LTC1174-3.3/LTC1174-5 where tOFF =4µsandVD isthevoltagedropacrossthediode. Note that the operating frequency is a function of the input and ouput voltage. Although the size of the inductor does not affect the fre- quency, it does affect the ripple current. The peak-to-peak ripple current is given by: I VV L A RIPPLE OUT D PP =× + () − − 410 6 By choosing a smaller inductor, a low ESR output filter capacitor has to be used (see CIN and COUT). Moreover, core loss will also increase (see Inductor Core Selection section) due to higher ripple current. OPERATIO (Refer to Functional Diagram) circuitry is shut off, reducing quiescent current from 0.45mA to 0.13mA. When the output capacitor discharges by the amount of the hysteresis of the comparator A1, the P-channel switch turns on again and the process repeats itself. Operating Frequency and Inductor Since the LTC1174 utilizes a constant off-time architecture, its operating frequency is dependent on the value of VIN. The frequency of operation can be expressed as: f t VV VV Hz OFF IN OUT IN D = − + () 1 S APPLICATI I FOR ATIO Inductor Core Selection With the value of L selected, the type of inductor must be chosen. Basically there are two kinds of losses in an inductor, core and copper Core losses are dependent on the peak-to-peak ripple current and the core material. However it is independent of the physical size of the core. By increasing the inductance the inductor’s peak-to-peak ripple current will decrease, therefore reducing core loss. Utilizing low core loss mate- rial, such as molypermalloy or Kool M µ will allow users to concentrate on reducing copper loss and preventing satu- ration. Figure 1 shows the effect of different core material on the efficiency of the LTC1174. The CTX core is Kool M µand the CTXP core is powdered iron (material 52). Although higher inductance reduces core loss, it increases copper loss as it requires more windings. When space is not a premium larger gauge wire can be used to reduce the wire resistance. This also prevents excessive heat dissipation. CIN In continuous mode the source current of the P-channel MOSFET is a square wave of duty cycle VOUT/VIN.Toprevent large voltage transients, a low ESR input capacitor sized for Figure 1. Efficiency Using Different Types of Inductor Core Material LOAD CURRENT (mA) 1 10 100 500 100 90 80 70 60 50 VIN = 5V VOUT = 3.3V IPGM = VIN CTX50-4 CTX50-4P 1174 F01 LOAD CURRENT (mA) 1 10 100 500 100 90 80 70 60 50 VIN = 5V VOUT = 3.3V IPGM = VIN CTX100-4 CTX100-4P |
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