Datasheet LTC3561A (Analog Devices) - 10

LTC3561A
APPLICATIONS INFORMATION
size requirements and any radiated field/EMI requirements the diode peak current and average power dissipation than on what the LTC3561A requires to operate. Table 1 so as not to exceed the diode ratings. The main problem shows some typical surface mount inductors that work with Schottky diodes is that their parasitic capacitance well in LTC3561A applications. reduces the efficiency, usually negating the possible benefits for LTC3561A circuits. Another problem that a
Table 1. Representative Surface Mount Inductors
Schottky diode can introduce is higher leakage current at
MANU- MAX DC FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
high temperatures, which could reduce the low current Toko A914BYW-1R2M=P3: 1.2µH 2.15A 44mΩ 2mm efficiency. D52LC Remember to keep lead lengths short and observe proper A960AW-1R2M=P3: 1.2µH 1.8A 46mΩ 1.8mm grounding (see Board Layout Considerations) to avoid ring- D518LC ing and increased dissipation when using a catch diode. DB3015C-1068AS-1R0N 1.0µH 2.1A 43mΩ 1.5mm DB3018C-1069AS-1R0N 1.0µH 2.1A 45mΩ 1.8mm
Input Capacitor (CIN) Selection
DB3020C-1070AS-1R0N 1.0µH 2.1A 47mΩ 2mm A914BYW-2R2M-D52LC 2.2µH 2.05A 49mΩ 2mm In continuous mode, the input current of the converter is a A915AY-2ROM-D53LC 2.0µH 3.3A 22mΩ 3mm square wave with a duty cycle of approximately VOUT/VIN. Coilcraft LPO1704-122ML 1.2µH 2.1A 80mΩ 1mm To prevent large voltage transients, a low equivalent series D01608C-222 2.2µH 2.3A 70mΩ 3mm resistance (ESR) input capacitor sized for the maximum LP01704-222M 2.2µH 2.4A 120mΩ 1mm RMS current must be used. The maximum RMS capacitor current is given by: Sumida CR32-1R0 1.0µH 2.1A 72mΩ 3mm CR5D11-1R0 1.0µH 2.2A 40mΩ 1.2mm VOUT(VIN − VOUT) CDRH3D14-1R2 1.2µH 2.2A 36mΩ 1.5mm I RMS ≈ IMAX V CDRH4D18C/LD-1R1 1.1µH 2.1A 24mΩ 2mm IN CDRH4D28C/LD-1R0 1.0µH 3.0A 17.5mΩ 3mm where the maximum average output current IMAX equals CDRH4D28C-1R1 1.1µH 3.8A 22mΩ 3mm the peak current minus half the peak-to-peak ripple cur- CDRH4D28-1R2 1.2µH 2.56A 23.6mΩ 3mm rent, IMAX = ILIM – ΔIL/2. CDRH6D12-1R0 1.0µH 2.80A 37.5mΩ 1.5mm This formula has a maximum at VIN = 2VOUT, where CDRH4D282R2 2.2µH 2.04A 23mΩ 3mm IRMS ≅ IOUT/2. This simple worst case is commonly used CDC5D232R2 2.2µH 2.16A 30mΩ 2.5mm to design because even significant deviations do not offer Taiyo NPO3SB1ROM 1.0µH 2.6A 27mΩ 1.8mm much relief. Note that capacitor manufacturer’s ripple cur- Yuden N06DB2R2M 2.2µH 3.2A 29mΩ 3.2mm rent ratings are often based on only 2000 hours lifetime. N05DB2R2M 2.2µH 2.9A 32mΩ 2.8mm This makes it advisable to further derate the capacitor, Murata LQN6C2R2M04 2.2µH 3.2A 24mΩ 5mm or choose a capacitor rated at a higher temperature than FDK MIPW3226DORGM 0.9µH 1.4A 80mΩ 1mm required. Several capacitors may also be paralleled to meet the size or height requirements of the design. An additional
Catch Diode Selection
0.1µF to 1µF ceramic capacitor is also recommended on Although unnecessary in most applications, a small im- VIN for high frequency decoupling, when not using an all provement in efficiency can be obtained in a few applica- ceramic capacitor solution. tions by including the optional diode D1 shown in Figure 4, which conducts when the synchronous switch is off. In
Output Capacitor (COUT) Selection
pulse skip mode, the synchronous switch is turned off at The selection of COUT is driven by the required ESR to a low current and the remaining current will be carried by minimize voltage ripple and load step transients. Typically, the optional diode. It is important to adequately specify once the ESR requirement is satisfied, the capacitance 3561afa 10 For more information www.linear.com/LTC3561A