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LTC1626 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1626 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC1626 sequence, the LTC1626 will delay entering Burst Mode operation and efficiency will be degraded at low currents. APPLICATIONS INFORMATION the P-channel switch duty cycle. At high input voltages, the diode conducts most of the time. As VIN approaches VOUT, the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the regulator output is shorted to ground. Under short-circuit conditions, the diode must safely handle ISC(PK) at close to 100% duty cycle. Most LTC1626 circuits will be well served by either an MBRM5819 or an MBRS130LT3. An MBR0520LT1 is a good choice for IOUT(MAX) ≤ 500mA. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low effective series resistance (ESR) input capacitor must be used. In addition, the capacitor must handle a high RMS current. The CIN RMS current is given by: I IV V V V A RMS OUT OUT IN OUT IN ≈ − () [] () 12 / This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst case is commonly used to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours life- time. This make it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Do not underspecify this component. An addi- tional 0.1 µFceramiccapacitorisalsorequiredonPWRVIN for high frequency decoupling. Output Capacitor (COUT) Selection The selection of COUT is driven by the ESR for proper operation of the LTC1626. The required ESR of COUT is: ESRCOUT < 50mV/IRIPPLE where IRIPPLE is the ripple current of the inductor. For the case where the IRIPPLE is 25mV/RSENSE, the required ESR of COUT is: Kool M µ is a registered trademark of Magnetics, Inc. INPUT VOLTAGE (V) 200 180 160 140 120 100 80 60 40 20 0 1626 F03 5.0 5.5 6.0 6.5 7.0 7.5 4.5 2.5 3.5 4.0 3.0 FIGURE 1 CIRCUIT 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 losses. Core losses are dependent on the peak-to-peak ripple current and core material. However, they are independent of the physical size of the core. By increasing inductance, the peak-to-peak inductor ripple current will decrease, therefore reducing core loss. Utilizing low core loss mate- rial, such as molypermalloy or Kool M µ® will allow the user to concentrate on reducing copper loss and preventing saturation. Although higher inductance reduces core loss, it increases copper loss as it requires more windings. When space is not a premium, larger wire can be used to reduce the wire resistance. This also prevents excessive heat dissipation in the inductor. Catch Diode Selection Losses in the catch diode depend on forward drop and switching times. Therefore, Schottky diodes are a good choice for low drop and fast switching times. The catch diode carries the load current during the off- time. The average diode current is therefore dependent on Figure 3. Operating Frequency vs Supply Voltage for Circuit Shown in Figure 1 |
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